If the land is privately owned and not stocked with fish by the local goverment, then you can do whatever you want with the fish. If the pond is overstocked and the fish are stunted, the ONLY 2 ways to clear things out is by A) catching as many fish as you can and not returning any, or B) introduce some big predators that will clean things up for you. Something like a pike or a snapping turtle would do well, though with the snapper, I'd suggest getting one of those water-proof, electronic dog collars so you can find and get rid of him when things slim down.
Or, you could throw a stick of dynamite in there.