the prior owner to our land was an avid duck hunter and whenever he saw a tresspasser he would go "hunting" and or practice his skeet shooting. and it seemed he had few problems with uninvited guests...he also befriended the right people ie game warden and sherrif.
We moved in and tried the nice guy approach even allowing a neighbor to grow walleye fingerlings in a portion of the pond. Before too long people were parking their boats on our land and would be out fishing at Midnight. It wasnt in our nature to shoot off weapons to try to scare them off but certainly the tough guy approach was more effective. We had probalby 50 or so signs all over the place and before too long nobody paid attention to them. We were only at the place about 15 times a year for 2 or 3 days so that didnt help. Our dog was a cute puppy.