This has come up in many threads, including one by me. The best advice I've heard is to block all entrances/cracks, and get rid of all ledges in the bays. While this can be difficult, if not impossible, it's the only way that I feel will work.
I've thought about cats, by putting them in the attic, however, I really don't want to deal with their smell. If I could only train them to stay outside at the wash, and enter the attic when on the prowl!