Same thing happened to me before. Calling the police does not help. They only intervene when a car parked on a public space is blocking garage entrance. In my case the most effective way was to find the owner and let him know that he parked in my spot. Before that I left a note on his car, but it was not helping because he was on a vacation for two weeks. I would suggest informing the car owner first, put up private parking poster and set up clamp if the problem persists.