I don't know, adding laws aside, of anyway to stop the bother. Maybe a polite call to the business that placed the add, explaining that you dislike them placing adds on your car. (polite phone calls can go along way)
I do have a quote to offer though.
"Remember that what is insulting is not the person who abuses or hits you, but the judgment that these things are insulting. So when someone irritates you, realize that it is your own opinion that has irritated you. Try, therefore, in the first place, not to be carried away by the impression; for if you once gain time and respite, you will find it easier to control yourself"