I think anemones move to whatever lighting and flow conditions they find optimal. Maybe yours is trying to get to the higher flow area near the output of the powerhead, but ends up getting sucked in the intake, which is probably nearby, right? Maybe you can adjust the powerhead to point more directly at the anemone's "perch" on a rock, and give him enough flow so that he doesn't move.
Another solution might be to set the powerhead in a position where the anemone can't get sucked in. Mount the PH farther from the tank wall and rockwork so that even when extended, he'd have to let go to get to the PH. It may be hard to rig a PH from the top of the tank, but you might prefer it to anemone puree.
Happy reefing,
Jon