I had a huge 6" One Spot Foxface that went through a huge collection of my zoas. I tried everything, catch boxes, fishing it out, and netting it several times but the only thing I was able to do besides tearing down my 165g tank was waiting till lights were out and spear it. I made the spear out of a old fishing rod tip and a straightened out fish hook with the barb glued into the end of the rod. it's actually turned out to be a really good tool for other uses in my tank since this!
I knew where it slept in the tank at night so it was an easy job of finding it and once I speared it near the tail I took the fish out the tank and cut the barbed end off and put it in a QT tank for monitoring it and to see if it would make it. It healed up just fine and I was able to pass it on to another reefer two weeks later.
If your careful with where you spear it the fish could actually live through this and you wouldn't have to tear your tank down!