Since Mitchel mentioned squirting some squirrely squirrels with water with some success, I'm reminded of seeing some motion-activated sprinkler that has been sold to discourage various animal intruders. I don't know of anyone who has used it, but it's got good reviews on Amazon:
Amazon.com: Customer Reviews: Contech Electronics CRO101 Scarecrow Motion-Activated Sprinkler
Boy, I'm thankful I don't have these critters!