If you want to help curtail toxicity, address the person being bullied, not the bully.
What I mean is, I see a lot of people getting harassed over stupid shit in this game all the time. Someone misses an ult, fail flashes, or any number of things that cause teammates to rage (omg, jg, why no gank? OMG, support, you took a creep! Raaaage!). Thing is, starting a back-and-forth with the person raging generally just causes things to escalate out of control. My advice? Be encouraging to the person getting targeted, and try and convince them from engaging the person being a dick.
Last night, I was in a game where our Syndra was losing lane, horribly, to a Lux. Jungler and top essentially started camping her lane to shut Lux down and bring Syndra back into the game. We won, and our Syndra, post game, started gloating and told Lux to refund her Elemental Lux skin, and trashing her. It's this type of behavior which can really tilt someone, getting them annoyed, and easily making that player, in this case Lux, turn into an asshole in further games because now they're mad.
All I did was assure that Lux that she had actually played really well, and not to let trash talk get to them, because they were giving us all a hard time; didn't bash the Syndra, or try to escalate the situation, and it seemed to work, because that Lux invited me into her group, and she was actually pretty cool.
Point is, toxicity spreads like a wildfire, with one person's rage quickly turning multiple people into trash talking dickheads. Best way is to jump to reassure the person being attacked to ignore it, mute the aggressor, and try and keep them positive. It might seem carebearish, but believe me, it beats the hell out of having 2, possibly 3 people screaming all game and causing you to lose an easily winnable game.