Riot has always said that retaliation is unacceptable, the reason being is because all it does is make a bad situation even worse, it does nothing to help at all. It doesn't matter if someone else started it or if the other person was being worse, you are only judged on your own behavior, not anyone else.
I do agree with this, to a certain degree. One is responsible for his own behavior. However, these instances do not happen in a vacuum. It does matter if someone else started it or were deliberately being provocative. This is how a lot of passive-aggressive jackass get away with so much, while their victims are punished for their retaliatory actions. Unfortunately, this is not the play ground or the basketball courts, where there are plenty of witnesses and immediate repercussions should someone try to browbeat his way over someone.
So how to deal with this? Well, playing it cool is the best way to tackle this problem, but easier said than done. Hey, there are times when a jackass deliberately tears me down and all my good gameplay, and I get ready to react in full force. These guys make it personal, rather than constructive or objective. They play with emotions, theirs and yours, in order to hurt someone. That is a key detail to remember. These bullies don't use reason or logic, but they will do whatever in order to hurt you in one way or another. It could be insulting your family, to playing Troll-nivia to get you killed. You need to be smarter than that, and being cool is really the optimal way to approach it.
I always try to remember the standards of debating and discussion (all those years in debate club should help you out now). If he gets off a personal remark, I try to just reply, "No, I'm not." Simple denial tends to end it right there. If the jackass is still insistent, then I reply, "Then prove it." That forces him to actually think rather than nerd-rage on chat. And if he can't support his claims, then openly dismiss him as irrelevant. You want to make this guy think as much as possible, despite his unwillingness to do so. If he starts thinking, he loses; there's nothing for him to stand on. So only the really stupid fools will persist in the harassment. And that usually leads to the inevitable "1v1 me, bro"
I tell them, "If you want to 1v1, then go play Street Fighter."
Just try to keep it cool. If you are defending yourself, negate any accusations against you, without having to attack him personally. Play it smart, put him on mute if necessary. Hell, if the situation gets really bad, then just leave the game. Sure, it will mark you as AFK or dodging, but your mental health is worth more than the abuse you would otherwise take from these jackasses. And if the other players recognize that the bully is the reason you left, then they would report his ass after the game.