I prefer to play long games without the pressure of having a clock that winds down quickly, but sometimes people will leave the game with their computer still open. Then when I finally have to leave or do something else I'll have to shut off my computer and lose the game. Is there anyway to stop this?

No, you should only play a game that can potentially last over two hours if you actually have the time to be there that long. If you have to leave before the time is over, you should have played with a faster time control.

And my condolences for that last game, I hate people who do that. It's very rude.

If they continue to do that, they may be restricted. Please see here: http://support.chess.com/Knowledgebase/Article/View/171/13/fair-play-policy

Cheers

Thanks that was just what I was looking for!