Thievery happens in the world often so it's never punished.
Murder and kidnapping happen a lot in the world so it's never punished.
A lot of something happening and it being punished can very much happen at the same time. Especially when the only punishment that's remotely feasible is easy as hell to work around. Especially if you're actually dedicating yourself to trolling and aren't just some raging manchild who can't control their temper.
Someone with diamond skill can easily make a new account, get back into diamond, then troll diamond games. Over. And over. And over. An actual troll - not just someone with the self control of a butterfly on 3 pounds of crack, but someone who is dedicating their miserable life to ruining people's entertainment - could do this easily. And there's no reasonable and feasible way to actually stop it.
Before you ask why Riot even bans them then - it's because it does drive some of them away. Eventually. It might take a hundred accounts, but eventually they get sick of it and go away.