I have a longspine, 2 pincusion(one white, one purple), and a blue tux urchin. The one you had sounds like a pincushion, too hard to tell without a pick. Look in my gallery, I have a pick of my purple pincushion stealing my nori clip. They can be quite mischievous.
Is it possible to keep them off the rock? No way, they go wherever they like. They do eat some of my corraline, but frankly mine seem more interested in the green algae. Maybe I am just lucky. Like Atari said, each is an individual, maybe another would do less damage.
I just happen to like inverts and am willing to put up with a little more than some. As long as they don't hurt my fish.