Selecting quit from the start menu should be all they need to look at. And then prevent people from jumping back into match making for 5 minutes, 10 minutes, and then an hour if it's done repeatedly. If someone wants to idle, give them the same penalty if no game inputs are pressed for 20 seconds or something. If someone idles for a set period in a match, kick 'em out at the *end* of it, and give them the same matchmaking penalty.
I imagine if people are forced to stick around for 2 minutes and lose, rather than be stuck at the main menu for 5, they might actually stop crying for a moment and pick the controller up.
Granted, policing the dregs of the internet is an impossible task. But at the very least, you can improve it slightly for those of us who aren't assholes. *shrug*
Erm, why not? All you're doing at that point is forcing the other guy to go through the matchmaking process again.