I had an infestation with hundreds at one point.
Got rid of all of them with distilled white vinegar in a syrringe with a needle.
I drew up the vinegar, and injected a small amount into the body of each anemone through the mouth, and they disintegrate on impact.
They do not grow back, except for those you did not treat well enough in the first place, and all you do is retreat them the next day.
I used this solution to do several a day until the whole bunch was gone.
You can also use Pickling lime (Ms. Wages) according to GARF.