Couple things:
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Yes, from what I've experienced, people can absolutely stack reports on you for the punish, and Riot can deny that all they'd like but hear me out. The first few punishments are definitely bot-issued, and you can tell the bot issues them when they pop up your restrictions while you're waiting in the lobby for the next game, talking about the game you just played like 30 seconds ago. Riot doesn't respond that fast. Those are the ones where stacking will get you chat restricted, Low-Queued, or 1-3 day banned for being "toxic" to the poor 4 man squad that can't handle being told that blaming their rando for everything doesn't mean they're actually better than their rando in any way whatsoever. From there, I think only the 14-day and perma bans are actually manual...but if the bot pings you a bunch of times, I don't think they really look into it fully: they see that during that game, more than likely after a few other bad games in a row dealing with ridiculous things, you said "f*** off" to the guy tearing you up in chat, and that F-bomb is all they care to see in order to throw you the banhammer, regardless of context, generally. My main got perma'd from it. To be fair, My main got perma'd from it because I'm not exactly the nicest person in the world when 2-4 people are all trying to pin the loss on me at 5-7 minutes in, with a better score than half of them, before the game's even CLOSE to being won or lost yet, and I'm not one to roll over and play dead for morons...but for all intents and purposes, my main was perma'd from stackers. Your best bet to counter that is usually to try and dispute it with Riot's support staff...or so I'd like to say, but to be real, the Reds in charge of these boards are 1000% more likely to actually look into things and attempt to work with you properly than the support staff are, or about 40% of the people who get perma'd probably wouldn't even be perma'd. >.>
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That all being said: don't be like me. If people seem to be stacking on you, the best thing you can do is legit mute everyone. If they start spam pinging you and being nuisances, mute the pings too. At that point you're not really playing League of Legends, you're playing "don't lose my account's honor, or my actual account, to a bunch of die hard pleb monkies wanting an e-peen contest." The moment you realize how the situation is, turn off all their input and play your own game. Don't say anything, don't try to play around them if you think what they're doing is stupid, if they've been trying to bait you and run, just switch to splitpushing and staying as far removed from them as possible if you have to, but give them nothing to use against you. Don't take that to mean not to play with them at all - if they're actually trying, try with them...but if they're trolling you, just remove yourself from them, accept the LP loss if you're not able to solo carry them, report them all, and move on.
I had my main from season 1...when at first that perma happened, it was a pretty big hit. I had every single Cait and Nami skin to date at that time, Hextech Annie...a lot of stuff. I really don't care enough to dispute it with Riot, as it's far too much effort, and as I said, I will own the parts where I fight fire with fire in the chat, which is by far more than enough for Riot to say I earned it. Believe me, I am NOT nice once I get going...and as much as I'd love all my skins and champs and most importantly all the time I put into my main back, I can't be upset at Riot for taking it away by making a nuclear chat offense my defensive strategy. Honestly, it'll be a matter of time...maybe another 6 seasons or so, before they perma this one. I'm not going to apologize for what I've said to those people, even if I acknowledge that it went way too far, because those people earned every word of it...I'll take the punishment from it, and hope those jerks get theirs along with me. Either way, bottom line is: if you value your account's well-being, just take my word for it, mute f-ing everyone, and try your best to clutch out the 1v9.
Now, in regards to your original question, the line for "competitive BM" is basically anything that directly calls out a player in a degrading way. "Anivia, is that all you got?" isn't really degrading. That's more like a challenge than a put-down. Something like "Anivia, you deal less damage than your support, lel" is considered degrading most times...even if it's factual. :P
Word choice is the key, basically.
