Since the guy can't be reasoned with, I believe I would tell him that I purchased the property and paid a premium for the ability to enjoy my property by fishing it. I believe that any reasonable person would understand that. I would then explain that I was prepared to take legal action to recover my investment/expense in the courts, including legal expenses. I would put this in writing and have it notarized with copies going to potentially interested parties. Talk to a local Real Estate lawyer to find out what kind of case law exists in your state to support or limit your rights to enjoyment of your property. There may be several gov't agencies that you can contact including the NRCS and County Agent. If the lake has any chance of overflowing and affecting any water holes downstream or might cause an environmental problem, the EPA might get interested.
If those don't seem to impress him, guarantee him a world class ass whuppin for every dead fish you find. That kind of personal explanation of the depth of your feeling might just get through to him.
