Mikel:
If you want to sugar coat it a bit, you could tell them that you are changing your goals for the pond, and to meet those goals, you have to implement strict guidelines for fishing in the pond.
That should leave it wide open for you to make a list of rules.
If you are actively managing your pond, you can go thru some $$, but more importantly, time growing fish. If it takes a couple of years to get fish to the size that you want, and someone comes by and removes them from the pond, what would it take to replace those fish in today's dollars? (that size of fish that was removed) That doesn't take into account the hidden costs of managing the pond - fertilizer, weed control, etc.