While it was pretty juvenile, and that's the key JUVENILE, to do that I kind of feel like it goes with the territory. I think if you're going to run a facility, then you're going to have stuff like this occasionally. I'd be grateful it wasn't permanent damage.
I WOULD do my dusting during their next lesson, to just sort of let them see who is having to clean up after them. I'd also schedule 2 or 3 barn days at the barn and have a potluck BBQ afterwards with some entertainment as a "Thanks for your help" kind of thing. If you don't have good local entertainment, then cater the BBQ instead of potluck-ing it.
I would not be shy about telling the instructor the next time her clients do something like that, that I would expect her to clean it up herself.