IMO, in an unstocked pond, algaecides are by far your best option!
Why would they be forbidden in your situation? Does water from you pond continuously drain into an adjacent body of water or wetland?
Even then, I would think you could block the flow of water into your pond, treat the pond, and then resume inflow (and outflow) after the proper time limit shown on the product label.
*** I am not an expert on NY law, nor did I sleepy at a Holiday Inn last night.
