What do you do about someone quietly, but deliberately, throwing?

Players AFK games. Riot institutes Leaver Buster.

Players verbally abuse each other. Riot put in place the verbal abuse auto punishment system.

But I just had a ranked game with a player who complained about a first blood KS, so he left his mid lane and went about farming the jungle (and frequently being executed). Never spoke another word all game. Team fights break out, and he casually walked away to kill wolves.

How can we deal with this form of abuse? Quietly throwing can't be picked up by the verbal abuse bot that reads chat. And throwing is one of the most impactful ways to ruin other players' experience. Rito please?