Well if some one is illegally on your property you can have them towed off but you will just be adding fuel to the fire here.  It is hard to say what to do with out actually seeing the lay out of the property and the parking spot.  But if it is hers alone she could fence it in with a gate; she could put a lock on the gate; put a garage, etc., all subject to zoning laws and permits of course.  Good luck.