When she walks in, 'borrow' her keys and in front of her, snap off the door key. Tell her the next time she 'walks in' you'll have her charged for B&E...
Park at the end of the drive so she can't get in??Or ask city hall what the limtations are on impound signage and the approved wording. Try to make it include Guests using the private drive, will be towed at owners expense... (Most cities want that in a real specific way)
Or as my old man would do, declare war on the roomate and tell him he has to get out and take his garbage pail with him.
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