I just encountered a Platinum player who has been intentionally feeding for the past 3 weeks

I'm assuming it's because he's still 'playing,' but in such a way that he intentionally makes his team lose. For example, we just had him playing Talon, and it was very obvious to everyone in the game that he was intentionally feeding, but he still got a couple of assists and CS.

He wasn't running down mid lane repeatedly, but he would randomly ult and just run through the enemy team. He wouldn't even attack anyone, he would literally just pop his ult and run past them. There were a few times where the enemy LeBlanc would jump on him, and he'd just continue CSing while she killed him.

Looking at his match history, I can see that almost every game in the past three weeks is like this (i.e. scores like 2-13-5 in a 30 minute game).

Can we get an explanation on why it takes so long for these players to be removed, and what is being done to improve this? Three weeks should be more than enough time for a player who's behaving like this to be identified and removed.