Focus on your own play
We've all seen it right, "Bruh, this Nami can't land a wave. Go back to bots dood." "Ohmygawsh, y is Kai building AP???" "GG man, Anivia is just going to feed all game..."
Literally dozens of games where the chat looks like this. But fun fact, this does not help your team win.
Encourage your teammate on how to do better, or say "hey, maybe don't go in without me next time", "lawl maybe 1v5 isn't so great xD", but it doesn't help anyone to be putting someone down so you can feel better about the fact that you aren't good enough to carry that game. (shots fired)
Simply put, the most important and game changing thing you can do is to focus on your own play. Any game I am playing where the chat looks like constant beratement I will tell them to please stop typing and focus on their own play and we typically can pull out a win.
I would challenge anyone to present to me a time when they were able to turn a game around by flaming their teammate?