If the land is not properly posted, they have to be warned once, same with a business, they have to be officially warned not to step foot on the property again, then it will be criminal trespass.
Our land is posted. Heavily (sadly). Posted signs are an eye sore but they have been up for years (and wiped clean, new ones replacing old ones, etc). Trespassers went past 6 signs when they went down our road. Worst of all is one trespasser lives on the county road that leads to the access road, and he has lived there for at least 15 years. I don't believe for a moment he was unaware he was on our land.