I haven't tried pvp yet, so probably not the best to comment, but wouldn't the fairest solution be that no one is able to leave a match by any method until someone dies?

This eliminates the problem of someone leaving and then rejoining for the quick kill, however if you lose connection mid-match it would have to count as a forfeit and death. This may upset players with bad connections getting a death everytime their conecction drops out however I believe you have more control over your connection quality (service provider/device choice, wifi/3g etc) than the other players actions. I'm no idea how often players drop out because of genuine connection issues but I think most would prefer to cop a death by lost connection (often the case with me in pve) and know that the other players can't resort to dirty tactics.

Make it a no way out, battle to the death!!