It really depends where your toxicity comes from. Are you flaming people when they play bad? Are you responding to people that are putting down you or your teammates? There are lots of angles where frustration can come from in League and basically every single one of them is viewed as being toxic. You basically need to not type anything even remotely negative in chat or you'll eventually be banned.
Personally I had a huge issue with saying stuff like 'We should surrender' or 'GG' or 'k I'm done with this game'. It didn't seem toxic to me to give up on games where people were trolling, feeding, not trying to win, etc. I eventually was permanently banned even though I never insulted a single person in the game. I just had the defeatist attitude on games that got out of hand and that was enough to be banned.
If there was an option to remove your Enter key as a keybind to open up chat that would be the best way to do it. Just don't type man. 99% of the time it won't help anyone in the game and you risk your account.