I apologize if this is answered elsewhere, but I couldn't find it. Thanks for any help.

I felt like playing some Realtime games tonight, so I joined a bunch. I spent a couple hours playing those that started, but now I'm ready to go to bed. But I am still signed up for several games that haven't started yet. When those games start, it's going to be annoying for the other players because I am immediately going to get booted.

Is there a way I can unjoin those games?

Click on each game, scroll down, and click the Delete Game button.

Delete game is only for those that actually create the game. 

if you've joined an already existing game go to the game info screen (for example: http://www.wargear.net/games/join/434269)

below the player list is a button that says 'leave game'
click that and you're out.