Riot, What does it take for a person to be banned exactly?

I'm really curious because can a person get banned for smurfing? Having too many smurfs? Buying accounts?

For saying extremely toxic things over the course of 1 day?

For being reported by a whole team more then twice?

Because spending a ton of time into this game and not knowing what can get you banned is kind of a scary thought. I would really love some clarity on this subject.