I usually take the rock the aptasia is on and burn it off with a small torch. That is if the rock is removable and there is no coral close to it. In your case i would get aptasia x or joes juice that was already stated. Good luck with the removal!
Thanks Cichlid-dude. May have to try that....the LFS didn't have anything for aptasia when I was there yesterday. May have to cut it off and try to get as much as possible and hope it doesn't come back