Actually, a powerhead pointed at it will work. Just be sure to use a small powerhead as too strong of a stream will risk injury. When I wanted a BTA to move from its current spot (it was attacking a neighbouring anemone ..) I did this and pretty much within the hour it had moved to a different spot. I used a 201 (small powerhead) and adjusted to its minimum setting.
Tickling them off a rock works too (although again, risks injury, but, ...). They attach so strongly that it can be a lengthy chore to get the whole foot to detach, espescially if the rock surface has a lot of texture. So definitely a little patience required.
