If they are on your property it's tresspass. If they are nibbling your nuts it is a more serious offense and you are allowed to disturb them by yelling things such as "Go away" or "begone you pesky critters".
If such stern warnings fail you should call your local police station and warn them that you are under attack by squirrels. They will probably send an ARV round. Taking the law into your own hands can never be condoned.