We had this problem regularly at a place I once worked where neighbours from the adjacent block of flats would occupy our parking spaces, despite signs pointing out that they were reserved for employees. One of my colleagues printed off a little leaflet and stuck it under the offenders' wipers. It announced that the offending parker had just won £1,000 in a radio draw and needed to call a certain number to acquire his/her prize.
When they called the number they received an earful from said colleague telling them if they parked in our spaces again then their motors would be towed away. It worked wonders.