I don't follow. If he quits at any point during the match, he forfeits the match and you get the win. There's no need to DG or warn him since he's only hurting himself.
And why would the guy winning complain?
I understand you just wanna play, but if the other guy is hell-bent on restarting, then that's his problem, and there's nothing you can really do except give him the loss. Anybody who ragequits often is not fun to play with, so why would you want to prolong it?
Obviously, this would only apply during a tournament setting, because there's no sense in wasting time with unnecessary restarts. If the guy does this during friendlies, then he's just a d-bag who isn't fun to play with.
EDIT: RE: new posts: restarting at the very end to save time is okay, I guess, provided the match is actually over. Generally speaking, though, I would just let it finish. There's something satisfying about seeing that green ivy around the big 1.