I do know that because I still see them in various matches from time to time. I don't know how Riot decides who should get banned and who shouldn't get banned, but I would like to assume that telling someone that they should die of cancer or that they should kill themselves would result in a ban- most of the time it just results in a chat ban which is ridiculous. In fact, their chat ban policy is pretty dumb; if someone is being toxic and you make a few comments back you are chat banned? That's akin to getting stabbed while being mugged, getting the knife away from the mugger and killing him with his own knife, but getting charged with murder. Pretty dumb. Maybe it would be a nice jester for Riot to specially tell us how they were punished.
The League of Legends community is one of the most toxic gaming communities in all of gaming, yet I don't see Riot doing much to stop this epidemic of toxicity. What happens when most of the team rallies to report another teammate for bad behavior? They mock them saying, "go ahead report me, nothing will happen!" and most of the time nothing does.
Perhaps these are just the woes of being a F2P game, perhaps if the game had a $39.99 price tag or a subscription it would get rid of these terrible players who have nothing better to do then be toxic.