Hi,
It can be very hard, they're a real pest. I've heard peppermint shrimp and Copperband butterfly. This is not well documented but Asfur angels are apparently excellent for this purpose. However, when you're dealing with a living animal you can never be sure so the best way is often to manually hunt them out and eradicate them. The best way seems to be to buy a syringe, fill it with boiling water and inject the Aiptasia with it. They are super fast so if you cannot inject it just inject the water into it's hole, that should take care of it.