Start handing out 1-7 day bans from pvp.
It's different if you go afk losing 10-400 in points, but people go afk in totally winnable games. It's not rare, actually happens quite often, that you take a beating at the start, then you start turning the game around. Going afk because losing first fight or in otherwise winnable game is just super kitten move.
Play to win, don't play if you're not playing to win. It's just mind blowing how often 1 guy ruins whole game for 4 other guys when there is absolutely no reason. Some games are hard and you really have to fight to win, going afk you a) don't give yourself chance to win and b) ruin it for your team. There really has to be some kind of action against players afking in matches, it's getting unbearable...