You've got a couple of options. If it is just a few, mixing some kalkwasser (a calcium hydroxide powder, which can be bought at the LFS or at the grocery store as pickling lime... look for the brand Mrs.
Wages Pickling Lime) into a paste and shooting the aiptasia should kill them off without otherwise hurting the tank. Also,
peppermint shrimp eat them, so getting one or two of them may solve your problem as well (though make sure you know what species you're getting, I think there are a couple that are sometimes misnomered peppermint shrimp).
