Banning = Effecting New Players
I convinced my little cousin, 17 years old, to start playing League (I had to give Runescape a spin in return) and he tells me that everybody is his games seem very well experienced and knowledgeable. When I checked his match history, I noticed that only 2 or 3 players (including him) would have "innovative" builds. I spectated 2 of his blind pick matches and outside of 2 clueless players (1 being him), it was like spectating a platinum match.
It's safe to say that any person banned will just make a new account, which creates an extremely unwelcoming, toxic environment for new players. There has to be some type of way to keep smurfs in check.