I'd go the trap route, but first off, do you have any idea how they got in there in the first place? Do you have a stream that might feed the pond in a high rainfall event? Was there water in a small pond when you built the current pond?
I have a small pond in my front yard with the same problem. I had it dig about 16 years ago and only stock it with BG. This is the first year I see bullheads in it. I have no idea how they got in there. No chance of runoff from any other body of water. I've heard blue herons won't carry them but I did have one hanging around the pond last year.
