I also have this problem on Regal tang(or Blue tang).
When its ~2~3" in size, no problem.
But as it grows, it started to nip at my LPS.
More feeding will not stop them fr nipping.
It especially likes zoas, brains & cinas.
Thats why these corals will not 'open' with this #@% nipping on it.
Only solution is to Remove them!!
They are very smart, not easy to catch/trap them.
What I did is to remove all the live rocks & catch the TROUBLE maker.