I've had good luck with a variation of ThePurple12's answer above. I use a power head with a hose attached to the output and use the hose to direct the flow at their foot and use the flow to 'peel' up their foot. It can take a while for each one (I'm manually directing the flow the whole time, I was averaging about 30-45 minutes per anemone with my last removal effort), but I had good luck doing this to remove several from my tank. This method works well, even when they're really into craggy areas of the rock (as mine were). I have found it helps in some instances to pull out the rock and do the work in a tub of water, being able to move the rock around helps direct the flow at the foot.