You could go after them with an aiptasia remover but that all has to be done by hand, injecting each one, and it looks like you have quite a few, but if you're patient it does work.
Depending on your tank size you could get either berghia nudibranches to eat them or a fish such as a butterflyfish or filefish. Or you could tear the tank down and do something drastic with the rock to kill them (ie. boil the rock, soak it in something that would kill them).
Sorry you've had bad advice, that sucks
