I can really sympathize. It's on a much smaller scale but we have had a similar experience at our vacation home. The house is situated on a two and a half acre lot carved out of a thirteen acre piece. It's on a small bay. For years there were just two cabins on the thirteen acres. The family that owned them was around all summer, but in the winter people would break into the cabins and squat. Others would walk through or picnic. After years of having to do major cleanup at the beginning of the summer they sold the thirteen acres to a couple who subdivided it into six lots, one of which we bought in 2002. However, before construction of any of the new houses began, people continued to hang out, especially on our lot which has a large boulder retaining wall down to the water. We knew nothing about this history of trespassing until after we bought the lot.
We realized the summer after we bought it and before we started building our house that people had apparently watched 4th of July fireworks from our property, leaving behind their trash, including fireworks debris on the beach. We cleaned it all up and decided we would come back and watch the fireworks there ourselves the next year. In the meantime, we heard that quite a few people liked to watch the fireworks from this easily accessible spot.
By the next 4th, the house was under construction. We brought over a grill and invited a few family members to join us. At about six o'clock, we went down to the primo spot and set up our lounge chairs. A few minutes later a small group of people came charging up to our spot and were clearly disappointed not to have gotten there first. They had come from about 40 miles away! We were not sure what to do, but hoped they'd notice the house and would realize they were on private property and go down to the beach. They did not and within an hour, literally a couple hundred people showed up. Many of them came up on the hill, trampling the vegetation that prevents erosion, pulling their beer coolers and pushing their strollers! We were shocked, as were our family members who are locals. Again they left behind trash, although someone came by early the next morning before we could get back and cleaned up a lot of it.
I spent many sleepless nights worrying about what to do for the next year. What if some drunk fell off the rocks and broke his neck? What a child got hurt or someone started a fire with a carelessly discarded cigarette? How could we get people to respect that this is private property?
The house was finished in April of 2005. Early last summer I had 25 rosa rugosas (lots of thorns, great spreaders) planted on the part of the hill that was most eroded. On the 4th, we invited our family again and also asked the neighbors we had met to come watch the fireworks with us. We put a rope with a No Trespassing sign across the driveway where people had driven up the year before. We set up tiki torches and a Private Property sign at the spot where most people came up the year before and spread out our lounge chairs. I put caution tape and Erosion Control signs around the rosa rugosas. Then I had my elderly parents and aunt sit in the spot we were staking out. I also turned on all the outside lights at the house so it would be obvious it was occupied. Oh, and we doubled our liability insurance, just in case!
Far fewer people showed up that year (I guess they HAD noticed the house the year before) and the ones that were there mostly stayed down on the beach, although a few climbed up on the rocks. I had to tell one kid to stop pulling smaller rocks out of the wall and throwing them down below when his parents appeared not to care what he was doing. The fireworks were fabulous as always and there was not much trash left behind.
We still have people strolling through the middle of property several times a week even when we're there. Our No Trespassing signs are ignored. Most people just pass through and some even wave, but we don't know them and can't understand why they think they can walk through private property like that. I was appalled on day last summer to look out the window by my computer and see an older couple on bicycles who had stopped in our driveway so the husband could urinate. I can understand if it was an emergency, but did it have to be in plain view about 75 feet from my window? Another time I was awakened in the night by male voices. When I looked out the window I could see that someone was in our hammock smoking something. I turned on the lights and they got quiet, but there were beer cans and rolling papers there the next day. The house is not small -- it really can't be missed. Didn't their mamas teach them better?
Anyway, I'm sorry you have to deal with the mess and the stress.
Norma