I had a pretty severe problem with them about 12 years ago.
When the population is huge, a biological solution is just to slow as the aiptasia seem to out breed the biological solutions, or it just takes to long.
What did work for me was injecting the foot of the anemone with boiling lemon juice. You can buy a syringe with a needle at most pharmacy's, you just have to ask. I would fill a coffee cup half full of lemon juice, microwave it for 2 minutes which made a rolling boil, then inject 10 or so in one area in a day. Then the next day do a different part of the tank. My tank was a 120g so it took me about a week to do the whole thing. Started over the next week.
By the end of week two the problem was under control enough that the nudi's could take care of it.
The whole process took less then ten minutes a day.
Becareful using to much lemon juice in one go as it can mess with your levels, but in my 120, half a coffee cup didn't hurt anything.
Good luck.