I tried everything mentioned above - they all killed individual aiptasia for a while but they always came back in greater numbers...
Finally I bought four pairs of Berghia (about the price of five bottles of Joe's Juice). After allowing the Berghia to multiply by laying many egg rings in a 2.5G tank with a bubble filter, I started introducing adult pairs into a 100G sump absolutely infested with thousand of aips. Within a month, the aips were gone. If they have no predators, such as wrasses, they are aiptasia eating machines that multiply like bunnies!
When my aips were gone, I passed my Berghia along to local reefers and more than recouped my $$$. I'm still harvesting Berghia juveniles from that sump, and I haven't seen an aiptasia in my DT or sump for months... I have to plunder LFS for aiptasia to keep the juvis fed...
So do it the hard way or easy way...your choice.
LL