Catch the fish, put it onto a shallow dish with some tank water (enough to keep it breathing but not enough for it to swim, should be lying on its side), and pick the bugger out with a pair of clean tweezers.
Side track... I wonder if anyone has tried this but instead of using tweezers to pry it off, use a syringe and jet spray directly on the isopod with freshwater? I feel getting the isopod to release its grip is less painful than peeling it off the fish. Thoughts?