Illinois now has the purple paint law. If you have trees painted with a 8" ring of purple paint its as good as "No trespassing signs" now. A couple cans of spray paint and an hour of your time and you'll be legally set for the next 5 years. If you see him again there, call the police and he can be arrested. A trail camera perched 15 feet in the air above where he parks will also be evidence.