OP,
You should ask them politely not to sit on your car.
If they do not response then you really need to beat them silly like kicking their head in. Your property and you have earned with your hard earn sweat and blood, so why should they try to treat it like theirs? It is not theirs.
However, be prepared to have your car damage in return, therefore, if you kick their heads in let them know next time you would kick them, their family, people around them and you would burn their house and their neighbour house down. After all, they can do whatever they want to your property then you can do the same.
😡