Mix up a solution of kalkwasser and either RO
water or saltwater, so that it's the consistency of thick paste. Get a syringe and suck some of the kalk
paste into it. Turn off all your pumps/circulation in the tank and then when the aiptasia is open, get the syringe down right on top of it and just squirt out a bunch on it, basically smothering it. Don't get this stuff on any corals/zoanthids. After about 10 minutes or so, turn back on your pumps.
The bugger shouldn't bother you after that.