My ex boss used similar tactics back in the '70's. We had a small unit on a trading estate and only had 4 parking spaces per unit. One particular guy from the unit next door wasn't very particular where he parked his car and regularly parked in our gaffa's spot. Despite regular warnings of what would happen if he continued to park in our spaces, he didn't seem concerned.
D Day arrived one morning when our boss arrived to find the offending car in his spot. He went next door and told the guy to move his car. Half hour later it was still there, so he went round again to complain and said enough was enough and there would be no more warnings.
After a few more minutes and no sign that the guy was going to move his car, our boss got in our forklift truck, picked up the guys car and put it in their skip.
There was a shouting match for a few minutes when the guy saw where his car was, but if it had come to blows he would have ended up in the skip with his car.
Never had a problem from that day forwards.