You can avoid jumping in on 18 hole games if you prefer not to by setting your preferences before joining a game. Scroll through the hole options (Front 9, Back 9, any 9, Full 18 or Any) prior to clicking the 'Find Game' button and you'll only join games that match that criteria.
It doesn't help any with avoiding courses you just played, but the previous answers should take care of that.
Kelsey