It really depends on WHY they quit. If something happened IRL (in real life), then I can understand that you do what needs to be done and the game comes 2nd. Another forgivable reason was just last night for me when a person who didn't know how to play joined and ended up quitting on the 2nd or 3rd round. I think the fairest way is to post on the forum, and then they have opportunity to defend themselves. Also not a bad idea to just avoid playing games with such a person.
I play a lot on BSW
http://www.brettspielwelt.de/?nation=en which is simply means "Boardgame World" in german. It's a cardinal sin there to quit the game before it comes to completion so I've gotten used to playing games till the end.