people don't kill them on purpose with city water, they start the garden hose, the phone rings and they forget to turn it off. Chlorine will kill your whole pond in 20 minutes. People generally don't do it twice. They are not happy to have killed their pond, especially if they have koi.
I go out and catch them but I charge a service charge if it is going to take too long. A local pond service or aquarium shop might even take in your orphans from the minnow trap.
Koi owners who want to put their fish on birth control generally stock a couple of catfish, they eat all the babies before they get too large. However if you only have goldfish they might eat too much, and catfish can get very large themselves. But catfish don't tend to reproduce in ponds.
