What is the punishment for sabotaging their teammate?

I got a troll today who sabotaged our team in ARAM on purpose because he thought it'd be funny. He was anivia and maxed his W first, going for no CS pre 10 minutes, firing his Q on his allies and always walling them off into the enemy once they took a single step to far from our tower. He did that consequently the whole game and we of course lost the game as it was a 6v4.

He got at least 4x reports for that game but I wonder what I can even expect to be done about him. If he was clean before doing that, is he still going to be permanently banned? The account was pretty low level so I guess it's a secondary account that is of no worth for that player.