That sucks.
I encountered the same thing last May while camped on Carcajou Bay. Two couples decided they would prefer to camp across from the falls (not a campsite) even though there were empty sites that I suppose weren't good enough for them.
A park ranger and his partner came by our site that afternoon and I ratted them out ... too bad. You should have seen the park guys when I told them, they were so pumped they hightailed it over the rocks with their motorboat
Had the park staff not shown up I wouldn't have said anything either.