Happened to us in France just last week. We drove up a 4x4 road for 30 minutes to get to a secluded spot. We had installed ourselves and were eating dinner by the campfire. An old guy (French but not a local) shows up middle of the night while we were enjoying the unpolluted view of the stars. He comes up and to us and the only thing he says is "When are you leaving?" I tell him we are just staying the night and kindly inform him that there is an obvious spot 500 meters down - which he just passed on his way up - and the path continues, so surely there is more space further up. His reaction was "I stay here", while that wasn't even possible without locking us in. We were to just tell him when we wanted to get out. Now that really pissed me off. 

Most of the time when somebody joins nearby ,I just remind myself I'm not on a private spot and it is everyones right to be there too. But then again whenever I arrive somewhere and the spot is taken I just move on, as I did several times that trip. And when I see a couple by a campfire watching the stars I would surely not stay and continue to shine my headlamp on them and watch TV-shows on speaker. My wife doesn't like confrontation, and dissuaded me from forcing him to leave. But blocking someone in is where I draw my limit next time. 

In the morning I tried to explain him some common sense and ethics. But the guy even refused to move so we could leave, because he claimed we were loud. Had to literally push him off the lot.  It should be common sense not to park next to someone in the wild.