I know it's a dick thing to do, but is it against any rules anywhere?

No, it is not against the rules.
Some gamekeepers might kick or ban you from official rooms for that though.

I'll let someone else speak to the legality of it, I'm only considering the ethics of it.
Losing is not so bad, your record is really not very important. How a person loses says as much about the person, as how they win. Be gracious in your loses as well as your wins and the game will be much more fun. If you steal someone's win by quitting or spectating out you will most likely make them unhappy. Do you want someone else to do this to you?

Actually the only reason I do it is when others do it to me and I play against them again.