I have the same problem, they came from my zoos and then have moved all over the place. I have zapped them many time with an aptasia liquid i bought from my local fish store. Everytime I zap them they seem to get stronger. They have started to kill my polps.
What makes me mad is I bought the zoos from my local fish store not knowing what it was. I had to buy the aptasia remover. LAst time I went to that store they had a huge rock covered in them, I asked the owner what's up do you like these and he told me they were pretty. I said maybe you should replace the corals I bought from you, because they are killing many of my corals.