Ok, this is absolutely petty as fuck, but keep a list of players who do this. If you get matched with them, send text to "all", saying what kind of player this is and for that reason you're leaving. My hope is that after a while, it'll be frustrating for that player to find a game, since once you leave, pretty much your whole team will leave.
Yes, I know it's not the right approach, but without a more robust system for dealing with toxic players, what're you gonna do
This is bad advice, especially considering the updates to in-game penalties and reporting for leaving. Not to mention the ease with which one can change their Steam nickname, it'll be difficult to manually track players that wronged you in the past.
At the moment I suppose the best thing is to report players in game. I'm not sure if the enforcement@relic.com email still functions. There is information about reporting players in the official forums, but I'm too lazy to find that for myself at the moment.