Honestly turning off cross-team chat was a really good step for me and shortened up a lot of my behaviors. Otherwise, it's a game-to-game thing. Some people just need to be told off and some people are just going to have a rotten attitude no matter what. That said, I've only been warned once in my time on League-- once in S3, by leaverbuster, for a disconnect from a bot game. It was tragic, but these things happen. You know?
The answer to your question is really just 'you don't.' Toxicity is the language of League, and they took away tribunal which was kind of curious. If you need tips, I might suggest that maybe non-toxicity isn't your thing. Lace your toxicity with more humour, let people know you're not there to impugn their family name.
Honestly, most of the times I've laughed my ass off at League have been toxic comments. My Lux game was off and my ults were bad, so someone told me, 'this lux here ulting like stevie wonder.' Good times. Give anything wit and humour and people will damn near make fun of themselves.