I have the same issue with a GSM. A real punk. She is at least 4" long and bites. She has knocked of stuff I would have never thought a fish could move. 2-3lb rocks are no trouble. She often picks up the new piece and moves it to the sand several inches from where it was on the rocks... If I hadn't seen it so many times I wouldn't believe it. And yes there is the occasional piece that they just won't leave alone... guess it means you have to another new piece so she has to chose which to go after first.
Sometimes it helps to have a "stick" (long grabbers or something that won't fall in when attacked) on the other side of the tank. That might provide enought distraction that you can at least get it placed. Then it's a fight to see who is better, you or the clown. I take the stick (in this case grabbers) and put them in the tank next to the new piece. Every time the clown gets too close chase her off with the stick. It could take several minutes of this, but it sometimes teaches them to not mess with it. Other than that... they really don't like things changed in the tank, so sometimes I will change a couple things at the same time just to mess with her. I've even rearranged half the corals just to make her leave the new one alone.