If someone really wants to quit the game, there's not much we can do to stop them.
We find a way to prevent them from closing the game while there's a match? They'll just unplug their modem.
Failing that, they'll unplug their computer.
Failing that, they'll turn off their house's electricity.
Failing that, they'll detonate their EMP bo
The obvious solution, as has been mentioned, is to simply reward the person who they were fighting - problem with that, as with many suggestions, is farming. Get a group together, go into a tourney server, everyone but one person quits and that one person gets the reward, repeat.
So providing an incentive to the non-quitter is out. I think the opposite - providing a disincentive to the quitter - is more reasonable. If people are ragequitting because they don't want to affect their W/L ratio, then maybe making a quit/spec in the middle of a match count as a loss for the quitter would help.
The counterargument to that is "But what if I'm in the middle of a match and I suddenly have to leave because it's past my bedtime/it's time for dinner/my whole family just got hit by a runaway aircraft carrier?" Well, the last one is clearly a case of mistaken priorities. As for the others, I think it's safe to argue that that's your issue, not ours. If you're going to commit to playing the match, then commit to playing the match.