Banning players from online games is joke you just make a new account and come back, my IP, Hardware change everyday.
However with that said Riot should allow toxic players to come back if they have truly been gone for a year or longer off League OF Legends would be fine, (However) if they come back and they start being (Toxic Again) Riot would manually Audit the accounts after so many games played and the players would just be (banned again) for another year.
Think of it as Probation, if they mess up and Riot sees it Banned again another year, however if the truly reformed then allow them to actually stay.
Riot should also make penalty much harder, as well as Role Selection Option for ranked matches, and help reduce the over-all toxicity. (Get rid of the automated system) and lay down (Specific Set OF Rules) instead of letting the community define toxic because to me toxic could be defined as talking about your household pet in a game to a friend.
I can tell you if Riot didn't have an (Automated System) or a (Tribunal) Players would actually be less toxic because.
. Players wouldn't spam threats to report each-other as much
. Role selection allows players to get what they want and less toxicity
. Role Selection allows players to open communication more when there are only two bot lane players who selected the roles its easier to talk to one another.
Role selection in champion select is one of the big issues right now from personal game-play because players don't take champion select seriously (Some do) But a lot of matches they just don't care and games go badly then harassment will start.