The report system does work, but for things that are not verbal infractions, it may take a few more games for the system to "prove" that the player is in fact breaking the rules. If you think about it, it's very easy for the system to prove whether a person is being verbally abusive, harassing, or if they are using hate speech, but it's much more difficult to prove whether someone is intentionally feeding versus just having a bad game.
And with instances of trolling, like this one, where a player was following the jungler around, it's much the same. While watching the game would make it easy to determine that the player is trolling, Riot simply does not have the resources to devote staff to watching replays of all the games that have players reported for intentionally feeding, trolling, or any of the other non-verbal infractions. So, rather than risk punishing potentially innocent players on the basis of false reports from angry/vindictive/trolling teammates, Riot requires a bit more evidence through the system before they issue a punishment for this sort of thing.
If this player has been doing this regularly, in all of these games and has been reported for it in them, he will be punished for it. But the important factor is that he has to be reported for it. I'm curious how you know that he has done it in all 20 games? Surely you didn't play all of them with him.
