He's making FFAs with a friend or multiple accounts and teaming up on the people who join.
I called him out in the middle of this game: http://warlight.net/MultiPlayer?GameID=3596656
You can tell by their picks and play style that they were teaming up to eliminate me so green could win. When I called him out on it he turned the game around and had red win to try and cover it up.
Blacklist them both and don't join their games.