If the aiptasia is isolated to one rock I recommend removing the rock and either using a paste made of kalkwasser, aiptasiax x or just plain lemon juice and treating that area and then using super glue or epoxy to cover the area directly where the aiptasia was... I have learned this thru research, many sites and people recommend this. With aiptasiax x or just plain lemon juice you will have to use a small insulin needle to inject liquid into the head.
also good info here;
you can inject them with lemon juice, aiptasia X or other similar pre-mixed chemicals designed for this purpose, kalkwasser, etc. and what I have read people have the best luck with especially on larger ones, is making the kalk paste. You make it nice and thick, slather it on the aiptasia, careful not to get any on anything you want to live, and just let it sit on there...when your sure its dead and gone add rock back to display tank...