I think we all want to give people the benefit of the doubt, however, it has been my experience with ponds and hunting grounds, that the majority of people wanting to use the grounds could give a sh*t about taking care of it.
If you've already seen garbage around, that is exactly what locals think of "using" the natural resources.
One realistic way to respond to people (which reflects some of what has been said) is to tell them that you need to get a grip or understanding of what you have existing in the pond, and until that time comes, you do not want anyone touching the pond. That sets a ground rule and an expectation.
This also allows you to gauge any individual's response. A true enthusiast would fully understand that, and might even want to participate. Only a true asshole would get pissed, and that's they guy who will be a future problem. Might as well address him firmly from the start.
