All right, let me have a try at this. People might be surprised how much of this write-up is based on personal experience.
It is also worth noting that, aside from the very obviously specific-to-UK examples (like the MotK Mafioso mention), this write-up can apply to everyone.
Mafia is an arguing-centric game, so it's only natural that people are going to get riled up. Lord only knows how many times it's happened to me over the years. The number one thing I think anyone can do when they get mad is step back. Go browse some other part of the forum, tab away from the forum entirely, or even move away from the keyboard. Take a deep breath or three. Relieve stress in a harmless manner. (Serp explained the background circumstances of that edited post to me - this is a good start! Just, yeah, as much as I'm sure you've already told yourself this, do your best to avoid that sort of slip-up again.) Most importantly, remember your win con. This is Priority One while playing Mafia. Not shouting some idiot down, not winning a moral victory. Getting town a team victory. You will do your win con better by responding to accusations with logic than with vitriol.
When you're a townie and someone makes a shaky case on you, 98% of the time, it's going to be due to one of these factors:
- The player in question is new and/or a relatively weak player. Getting mad at them will only serve to make them not want to play more. This works against your win con. If you go about dissecting their case logically without being snide or using rhetoric or insults, however, they may be internally upset that their case turned out to be wrong, but they will be upset in a way that spurs them on to find a newer, better case, and to improve as a player. Veering townies away from cases on town AND inspiring them to play better works toward your win con.
- The player in question is scum (or otherwise anti-town). Getting mad at them will show scum you're shakeable, and even worse, it may encourage townies to start looking in your direction, regardless of how otherwise sound your defense is. This works against your win con. If you go about dissecting their case logically without being snide or using rhetoric or insults, however, the rest of town will be able to see how smoothly and easily their case had holes poked in it. You will look better as a result, and they will look worse for making such a bad case in the first place, not to mention they have the "bonus" of trying to find a new person to BS a case on while they're under heavy scrutiny. This works toward your win con.
- The player in question is choosing between you and someone that looks better than you in LYLO. If this happens and it leads to a scum victory, there's nothing to be done except tip your cap to scum for a well-played game, and then spend the rest of postgame going over why you were chosen over them and seeing what you can fix for the future. Mafia is a game, but it's a social game, so good sportsmanship is important. And, most of the time, the person that makes the wrong choice in LYLO is going to feel really bad about it (all the more reason not to rub it in that they made the wrong choice - personal experience speaks on this one), and will be more than open to figuring out why they made the wrong choice and what they can do to avoid making that wrong choice in the future.
- The player in question is ignoring their win con in favor of carrying out a personal vendetta against you. If this is the case, you can simply ignore it, as they will be slammed hard for it after the game. Or you can bring it up with the game mod and have them talk to this person. If that person refuses to back down, ignore them and let the mod handle them. I know that I don't want people actively being anti-win-con assholes in my games and will gladly remove any such offenders as I please.
Then there is the occasional oddity, like an Insane/Paranoid Cop getting a Guilty return on you, to which there's nothing you can do but shrug your shoulders postgame and go "it was part of the setup". Or maybe someone is just simply flat-out mistaken, to which the bullet point about the new/inexperienced player largely applies. Getting mad at an experienced player over a case is more likely to make them want to hold on and not let go, which hurts both of you (and your team and win con as a result). Everyone makes in-game mistakes sometimes; it's best to just let them go after the fact. (Imagine what it would have been like if bofh had gone on a public multi-paragraph rant about how stupid you are after MotK Mafioso, just because you made a mistake.) No one wants to be on the receiving end of one of those, especially when they know they made a mistake.
As you can see, in all of these examples, going the logic route instead of going the vitriol route either works better toward your win con (the "game" part) or works better toward your social interactions (the "social" part), and it does so without risking damage to the one it doesn't actively work toward (if there is one).
What specifically can you do to quelch vitriol? I'll point again at the post you edited away, and the purpose behind its existence. That's a good start. Private, away-from-the-game stress relief can work, even something as cliche as punching a pillow. There are other things you can do, too.
- Don't call things "stupid", "retarded", "bullshit", "fucking [negative adjective]", or the like. No one likes being treated like that. I'm sure you don't. Instead, try things like "This is just silly, as [fairly obvious core Mafia reasoning]" or "This is factually incorrect, as can be seen in [post number/link]" or "That is not what I meant, and by looking at [post number/link], [post number/link] and [post number/link], you can see it is pretty clear what I DID mean, which is [reiteration of previous stance]". If someone is wrong about you, don't tell them they're wrong, show them they're wrong.
- Remind yourself that it's just a game, and that humans are liable to make mistakes. No one is going after you personally (aside from maybe RVS but that can be brushed off as lolRVS), and if they are they will be dealt with either during or after the game.