Dave good point about rainfall. If you do not want to drain, I have luck fighting off green sunfish in ponds where hybrid bream were stocked. The ponds basically had some pure green sunfish in the ponds and we stocked coppernose bluegill in them (maybe some can help with the explantation) but I feel the reproductive success of the bluegill helped the situation and did see any greens after about a year. So consider stocking bluegill and pulling out every green sunfish you catch and release all bluegill back into the pond. This along with prayer may get you out of the situation but it will not be easy.
