I would do what ecurB said, call Encon and speak with the officer that covers your hunting spot. Even ask to meet him in the spring to walk the property to get his opinion. Then assuming he agrees that there is no "high water mark" because there is no tidal influence then tell him that when the water fowl season starts in the Fall you are going to be hunting there and also monitoring the spot. If people that don't have permission continue to hunt there call the TIP line or the Encon officers private cell phone and have him come and issue citations.
I would also post the land and add an additional poster that says the land is not tidal and the shoreline and bottom of the water body is privately owned. Once it becomes clear that the private property rights will be enforced then the word will get around and it will stop.