I was talking to my LFS the other day and he suggested hydrogen peroxide dips. I believe I've read on here people doing them with success. I tried it once and never did it again as it's also a really hard dip on the zoas.
If you're interested, he suggest, a gallon tank saltwater with 1 liter bottle of hydrogen peroxide. Stick a large powerhead in there to stir things up. Dip the zoas for 5 mins and return to the tank.
I'll do a quick search before taking my advice. Just my .02