What do to when you tell someone to stop doing something and he keeps doing it.

In my case I told the guy almost 5 times to stop touching my car(he was my schoolmate) and only on the 6th try when I was super serious he stopped.He was putting his feet on my car and shit.

Now this is a highschool environment and I'm not some scrawny kid(18,185cm,90kg) but people still do shit like this to me.

How to proceed with these types of people and how to make people respect my boundaries more?