ban requirements?

in my games i see ppl going like go die, your mom died, you no mom, etc flames i report them but doesn't seem like they are getting banned. maybe because the player didn't tell me directly? I was in 3rd point of view, someone was saying it so someone else so I reported the person but no punishments. Also, I know after match screen chat is reporte-able too but ppl who are really toxic doesn't seem to get punishments? Riot should really start working on those bans. Doesn't seem fair where I dont wanna get banned so I dont flame, and there are people just flaming but no bans.