Peppermint shrimp will eat them.
I've got a few in there, as well as a couple other species I've heard may have an appetite for them. I'm actively searching for more, and plan to add a few that I've found.
I haven't seen my peppermint shrimp touch them, yet. Still spying on them to try to see if they will. Either they, or the copperband butterfly, do a phenomenal job on the aiptasia - the refugium is crawling with them (either due to the marco rocks, the Tropic Eden live sand, or the Miracle Mud, all three of which were in the tank for a good long time before any livestock was added, but which started growing aiptasia all by themselves), but the display is completely clear. If they were big fans of the hydroids too, I wouldn't expect to see the population in the display grown as large as it has.
So far, I've seen emerald crabs pluck them, taste them, and spit them out (probably couldn't hurt). Bought a bunch more emeralds to see if that will help.
I've heard good things about blunt-end sea hares (Dolabella Auricularia), one of which I had for a while mowing down what I thought, at the time, was hair algae. I'm going to get another one and watch carefully to see if he goes for the hydroids.
Flame angels, according to some, may develop a taste for them as well.
More obscurely, I hear that the lynx nudibranch (Phidiana lynceus) has an appetite for some species of hydroids, and I've got my LFS guys looking, but finding one seems like a longshot.
