Try 2 nets. 1 BIG, 1 small. Set the big net near the glass and use your hand or small net to drive the damsel into it. I would catch is now rather than letting them sort it out. That, or get a sea anemone, that will take care of the clowns. If it is an open top tank it doesn't take much to make a clown jump - like a damsel attacking from below.