In FPS's [First Person Shooters] spam has a few characteristics that distinguish it from what would be considered normal aiming and firing. Firstly you aren't firing your weapon, or using your ability, to hit a specific player or hero, but rather to deny a certain area or damage those who do not respect the incoming spam. Secondly, this implies that you are either firing continuously (i.e. Pharahs rockets, or Junkrat grenades), or you are firing a large volume of rounds (i.e. Soldier: 76, or Bastion in Recon mode [Implicitly the difference is you can't always hope for spill damage, so the fear of damage comes from the number of rounds fired]).
When do we spam V.S. Focusing a player/Hero
Here we want to first establish better and less than ideal times to spam, so when is that and what conditions are we looking for? There are definitely some easier times to decide when spam is a good option. For instance, if the game has just started you can spam the area where the enemy team will be leaving the gates (if you are on offense and its an attack/defense map). If it is a control point or pay-load map (Hanamura, Dorado, Hollywood, etc.) you can spam as the attacking team prepares to advance on either a Control-Point or Pay-Load. We especially want to consider spamming if there are a limited number of paths to a given point.
As illustrated to the right, at the beginning of kings row teams will sit behind the statue in the middle of the large red rectangle and spam rockets or grenades into the oncoming attackers. Because there are only two ways into the control point [Both illustrated with red boxes](unless you can fly of course), spamming here is uniquely powerful. One might ask at this point, because we have only discussed how the defense should spam, when should the offense? The answer here is much more straight forward, because you should primarily be spamming when they have a strong position on defense. Naturally if their defense relies more on spreading you out and picking you off, focusing is going to win out. But when they are relying on heroes like Torbjörn or Bastion to hold the line, you can spam these two (and in Torb's case, the turret) from a safe distance to avoid their retribution.
Like when mentioned above for defense, you do not wish to spam in situations where you need a specific hero down (i.e. you need to kill their Mercy in the teamfight before she Resurrects her team). Another important time to focus your fire is when ultimate abilities are being used (Killing a Pharah in the first second of her ultimate is game changing, or destroying a Junkrat's tire before it can wreak havoc upon your team). Importantly all the above examples share that they are intrinsically tied to teamfights. You naturally do not want to fire at a small red box when the enemy team is charging at you. So use spam effectively before the fights break out, and you will have the upper hand.
We are spamming like Twitch chat, but why?
First it forces them to avoid that area (you have now denied them ability to go there, thus any benefits from being able to go there). This is especially powerful when a Widowmaker who has been harassed out of her position wants to setup near the heat of the action, or in the above picture: at the bus. The untargeted death you have been throwing out might again force their Widow to do nothing while she finds another angle.
Secondly all this damage is naturally going to force their healer to move to heal the heroes that have been hit. This will stop a Mercy from damage amping her team, or stop a Lúcio from speed boosting his team into better positions. There are more effects on healers specifically, but doing this is already a big win for you. Taking all this into account, remember there are plenty more benefits to effective spam, but the list is far to long to write here. Because these effects occur before the fight, it increases the odds of you and your team winning the fight.
Consequently Winston is forced to leap forward without ult to create space for his team to advance and doing so he dies. Quickly after this the McCree is naturally out of position and dies. With these two picks, the ensuing fight goes back and forth, but the momentum from the initiation carries FlatEarth through the fight and advances the payload.
It should be mentioned here again that the ideas of what spam are, when, and how to use it, are loose at times. This is because the surrounding ideas come with conditions that you will learn by playing and experimenting. With that being said, I hope these ideas help would help you spam away the competition in your matches. I hope you all success in your deployment, and may all our Watches be long!
Working list for Heroes
Best at Spam (The order here for me does particularly matter for how impactful their spam can be).
Tied for 1. Pharah, Junkrat, Torbjörn : These are tied for top 3 because both Pharah and Junkrat have explosive weapons (Pharahs straight shooting rockets for better precision and Junkrat for lobbing behind walls and such). Torb I put here because his primary fire on his gun is a lobbed shot that doesnt loose much damage over distance. As well, his turret is free spam whenever the enemy gets in range.
2. Genji : His primary and secondary fire are great for spamming, great for damage, and he can position better than most heroes to do such.
3. Soldier: 76 : His rifles RPM and reload speed being fairly quick allow him to fire a decent volume of rounds into a given area, and he comes equiped with a "grenade launcher" (although its more a rocket launcher.
4. Zenyatta : His orbs are easy to throw out in high volume, he can charge up to allow a larger concentration of fire in one area. Lastly his damage does not degrade over distance.
5. Zarya : Her second fire being a lobbed energy grenade allows her to deal out damage around corners and at a decent distance. Additionally when she is a higher charge, these energy grenades can hurt a lot.
6. Lúcio : His prirmary fire is ideal for throwing non-target "bullets" in a general area. Because his shoots are slower moving sound waves he can easily spam chock points, capture points, and payloads.
7. Bastion : Hes everyone's worst nightmare. If he gets set up in Sentry configuration, the amount of bullets he can rain on your enemies is disgusting.
8. McCree : Bottom of the list because ideally most of your impact comes from strong usage of flashbang, and fanning the hammer. But his primary fire with his Peacekeeper is 100% accurate, and great at poking the enemy down.
Alright at Spam (Order here does not matter, each hero is here because they have limitations to spam compared to above heroes).
1. Tracer : Because she has two SMG's (Small Machine Guns) her damage degrades over distance greatly. Not only because of the kind of bullets, but also because her accuracy falls through the floor at any significant distance.
2. Roadhog : His secondary fire allows his shotgun to gain a little bit of distance making him an alright hero to be spamming. His primary in a closer quarters map is definitely ideally for spamming down rooms of squishies.
3. Reaper : Kind of the same reason Roadhog is down here, although his shotguns do allow you to put a volume of fire into a smaller room, or inaccurately a larger room, you mainly want to be focusing down isolated targets.
4. D.Va : Once again same kind of logic as Reaper and Roadhog, her primary attacks in Meka form are a shotgun type weapon and lose accuracy over distance greatly. However in her human form she is quite decent at laying down some spam.
5. Hanzo : Mainly down here because you dont want to really spam his arrows. Although you can sort of spam Scatter arrow, you really are looking to be a percise sniper for your team and make your shots count.
6. Symmetra : Her charged up orbs are the reason she is here and not below. When you are not under immediate threat, or your team is not being pushed in, you can throw plenty of these annoying death balls slowly rolling into your enemies faces.
7. Winston : Mention here, because up close he does effectively spam lightning on all your friends and family.
8. Mei : I mention her here because her Icicle isn't that bad at poking at distance, or just covering a smaller choke point.
Avoid Spamming (Order again does not matter)
1. Mercy : You can when nothing is happening with your pistol but you 99% of the time should be amping someone else.
2. Reinhardt : His one option to really spam at distance should be saved for key combos / hurting the squishier heroes.
3. Widowmaker : I mention her here because unless you feel very skillful with her, you won't want to give away your position consistently.