Ya i would agree that if you dont want to go buy a medicine to treat the with, boiling water in a syringe might do it. Make sure not to get any of the hot water on your surrounding corals though. I have heard of using lemon juice, acid, and other things to kill them but i think hot water will be way safer in comparison.
The problem with aiptasia is that if even a little piece of them remains alive they might come back. They can spread terribly fast in some cases so make sure to get rid of them soon,
Mike15.