Riot Banning people
Can someone explain me how the system ban you? 
12 Comments
https://support.riotgames.com/hc/en-us/categories/115001242847
If you're looking for a more specific answer than that, you'll have to narrow your question.
https://support.riotgames.com/hc/en-us/categories/115001242847
That's a pretty good resource. They may have it in different languages if English isn't yours.
there are bots.
one is a bot which scans the chat you said for negative words. the words that get you banned include insults and negative things, like "ff20", "we can't win", "bad team", etc. say that stuff over multiple games and get the ben hammer.
the other is a bot which scans your KDA and your win percentage. if you have a low win percentage, low KDA, and get reported for inting a lot, it gives you the ben hammer. this bot also handles trolling reports, but I'm not 100% sure what it looks at to do so. getting reported a lot matters, I'm sure.
Talk shit, get hit.