In order to deal w/this problem, you need to become a bat.
There are TONS of resources on the WEB that talk about getting bats out of the attic. I did a WEB search on bats in the attic and found 110 links. One is: Bats in the attic. You have to time the work to their habits. You DON'T want to catch them when they're breeding, because you will catch the parents but NOT the 'kids'. So, what I'm saying is that you have to educate yourself about their habits. You'll have to plug all the openings in your attic, catch the buggers, and so on. Consider putting a Bat House outside so they can live in it. Bats are actually GOOD to have around. (Read up on that too because placement of the house and such will either make the house 'attractive' to bats, or not.)
You could hire a professional but, again, they may catch the parents and you'll end up calling them back for the kids! My best to ya and hope this helps.