I don't know about kill but they are definitely a threat to its well-being. My briareus is pretty large but she reacts very quickly if she bumps into an aiptasia. If you have a lot of them buy a small jar of Kalkwasser, it's cheaper than a tiny bottle of Joe's Juice and will go further. You can make a paste out of it, about half kalk and half RO water. Get a plastic syringe to inject it directly into the aiptasia's and they will melt within about 30 minutes. Once they are in your tank you'll have to repeat the process every couple of weeks. Even though they seem to die off there's always a stragler somewhere that will reproduce.