A good easy way is to direct flow directly at it..This should eventually un-nerve the nem, make it un-happy with its location and then start to move..That is then your time to move it yourself when its mobile..
You just have to make it unhappy. The powerhead trick should work, if not then just iritate it by either shutting the lights off for extended period of time, killing flow, taking a knife and cutting it off... JK!
Rubbing ice on the foot some times works as does a credit card or even taking a chisel and scraping the liverock below the foot being careful not to hurt the foot to pry a portion of the foot off the rock so you can slowly peel its foot off. I was lucky enough to attend a presentation held by Anthony Calfo last weekend and watched how to clone an anemone by cutting it and he used the chisel method. I cant wait to clone my first nem, lil nervous tho its a dinner plate sized bta and I certainly dont want to make any mistakes and loose it...