They are next to impossible to catch with a net, but if you geta clear acrylic box in one corner of the tank you just scare them into the box with your hands and scoop them out. Works like a charm.
If you don't have a fancy clear acrylic box, like I don't, just use a big mason jar or similiar. the fish don't really "see" it and if the jar is backed into a corner with the mouth facing out along an aquarium wall the fish will scoot rigt in and wham.. put your hand over thoe mouth and it's relocation time for mr. PITA damsel.