There are two things that you can do.
1- If you have proof he did it, have him pay for it. This should be relatively easy to prove, especially if you have designated spots.
2- Park away from people and walk the extra distance, I do this all the time... and I enjoy the crap out of my car.
3- (bonus) do nothing and take steps (like #2) to minimize your chances of careless people damaging your car.