And the clear coat and paint underneath it!

Oil based paint thinner is another story though. It doesn't hurt clear coats and will sometimes remove spray paint or over spray if it is fairly fresh.
Alot of cities use a graffiti paint remover that is based on orange oil. It works very well if you remove the spray paint within a day.
One example >>
http://www.taginator.com/
If a vehicles paint has a good coat of wax on it, spray paint usually comes off rather easily with polishing compound and a buffer. I have a friend who is a detailer and it is amazing what he can do.