We had the same situation. My wife's office is at the front of a 5 acre tract, using perhaps 1/2 acre of it. The 5-6 neighboring buildings are the same, and the back of all these properties backs up to a power line easement that's about 200 yards wide. So a lot of land with no people.
I've taken my kids to ride dirt bikes and play paintball in those woods/scrubs for several years. This spring, while walking the land prior to playing paintball, I came across 2 tents in the woods, almost exactly the way you describe them. Evidence of recent habitation, but nobody there (at least that I could see, no telling who might have been concealed in the underbrush).
I took my kids out of there ASAP. Then I called the county sherriff. They sent out a deputy, I walked with him to the tents. He took them down (not very gently), threw them in his trunk, and marked the area with crime scene tape.
He said that when the homeless people returned and saw the crime scene tape they would leave the area. I asked about their tents, and he said they'd thow them away in a few days if unclaimed.
I went back a week or so later, and didn't see any evidence of re-colonization of the area.
For me, the answer was to get the sherriff involved. However, i felt like crap that these peoples few belongings were taken from them and they would probably never see them again.
You can leave a note telling them that you will call sherriff if they're not gone in in a day or so... Don't threaten, don't give name, don't even say you're a neighbor. Then call sheriff's office.
My guess is that your county police will react about the same way.