When is trolling going to be bannable?

Just played a ranked game where we were clearly winning and could've definitely took the game but, when Heimer got criticized by a teammate (it wasn't even flamed) he decided to troll and int. At first he just ran right into them and died. If he continued this it would've been an easy ban. He stopped after once but then just sat in the side lanes farming while they took our base. Even when it was clear they were coming top or bot he would stay and just die. The sad thing is, that he wont get banned. Riot just sees it as him having a bad game or just not paying attention to the map. It is very clear that he just all of the sudden started playing way worse and it was even intentional. I know it wont get him banned but it's annoying how one player can just lose you a match and go unpunished for it.