If you had noticed at the time, refusing to pay the bill until they agree to have all damage repaired , certainly would have made them be careful NEXT time...
But seriously, how long was it before you noticed? If it was only hours, go back complain, it's your property they're working on , you're paying them, there's an expectation that they don't damage things...
It's a shame to have to find out this way, check out ANY work done on your car BEFORE you pay. Most mechanics, etc, are careful, but some are not.