Well, for starters, you can physically try and remove the bat, or pest control companies can come out and spray the attic with bat repellant. (smells a LOT like moth balls, probably the same stuff).
When we got our house sprayed, I had to wait a couple days after the application, which was supposed to give them time to vacate, then plug the areas where they were getting in. The point of access is your real issue, though. It'* amazing how small of an opening they can get thru. Once inside, they crawl around just like a mouse.
It was interesting, as about a month after I screened over the vents that they were using to get in my place, I saw one flying around outside, and he went right up to there. Landed on teh screen and couldn't get in. But it remembered that is USED to get in that way, even after being driven out by the repellant. I suppose the smell wears away after a while, and they figure it'* OK to come back.