do a pre-emptive strike.
have your father contact the cement factory. Have your father state to the effect "I am the property manager of the radio tower. I have been informed that one of your employees has been tresspassing on the tower grounds. I was told that the employee's boss was close by when this happened. Do you have any information on this? Please tell all your employees that they need to call me at XXX-XXXX and ask permission before entering.
Then once you have the name of the individual who tresspassed, have your dad conact him and basically say. "Sir, I have some responsible shooters who carefully use the radio station land for shooting. They tell me you claim to hunt nearby. As you know sir, a rifle bullet can go quite far. Please be aware of the location of our property while hunting. Note that we will inspect our property for damage done by hunters firing from their property onto ours. Would you please consider not hunting close to the radio tower?"
Throw him off balance, and get him in some heat at work. No way should an employee and boss tresspass on another person's property and level demands against the legal users. The fact that the boss was close by, now the cement company is going to wonder what is up, and they are going to put pressure on that employee to not screw around and make more trouble because they don't want to get dragged into it.
See, this kind of thing cuts both ways