I agree that most public parks that allow fishing should have restrictions in place to NOT allow the removal of any fish. Most of the ponds that I fish are in fact that way. I do know some that are not. I would head to the closest lake or biggest body of water to catch and remove any fish. This way the population will not take a huge hit if I remove 20-30 sunfish. You take that many out of a local pond and it will take years for it to fully recover depending upon the size. Also you could hurt the other fish population like Bass and Catfish. If they don't have food to eat, they will die off.
The key is balance. Taking a few won't hurt anything, taking 20% will.