Stop banning for chat, start banning gameplay
(Note* If a player's chat logs indicate toxicity simply de-activate the allied chat and all chat options available for them so they don't get the options of seeing chat. This makes sure that if they troll via in game then they are banned but can still play without just buying a new account like everyone does)
Pretty sure I speak for everyone in the league community here:
Stop banning players based on their chat logs AND
START BANNING PLAYERS BASED ON OBVIOUS GAMEPLAY GRIEFING!
That malzahar top lane that seen you take his cannon and push the lane so it bounces back to him is mad and is now taking your krugs, going bot lane while opening top to take your gromp and wolves? Ban that guy. Especially if his chat indicates he's intentionally feeding and he goes 2-9 from 2-1 and his chat says "I'm gonna make sure you win"