The best way to keep them out is to find out how they're coming in, and block the entrance with steel wool or something they can't chew through. Most of the "urban legend" deterrents (dryer sheets, ultrasonic repellers, etc.) don't work according to industry professionals. Maybe that's just because they want you to call them to come spray some kind of poison, which doesn't sound too good to me either. Set a couple old fashioned snap traps in there (peanut butter works well as bait), and once you get a few of them chances are they will stop gathering there.