Riot, the only way to stop toxicity is to remove the in game chat
If the in game chat was removed, players could not harass each other at all. Pings would be enough means to communicate (provided that a few more are added), and people would learn to read pings. This game is known as one of the most toxic games out there, and even though I know you guys are doing everything you can to stop it, this would be a huge step.