Have the same issue on my property in Northern Michigan. It's accessable only by seasonal road (very disbursed camping) but people come in on the seasonal road and drive into the (clearly posted private property no trespassing) part and party, do drugs (I suppose) and leave their trash for us to clean up. Happens all the time. People are their own worst enemy. Every time I go up, I haul out others trash. I pay a lot of RE taxes on the land that others abuse.
Guess that's why I always carry a sidearm up there.....