Back in '72 in my only trip to Yellowstone, I spent most of the day chopping firewood and stacking it between two small trees. At dinnertime a church youth group came in and set up next door. Soon three teenagers were helping themselves to a big portion of my labor.
I informed them that they were stealing, but they complained they didn't have any firewood. I told them to take what they had and never come back. End of story.
Last year at a CA State Beach the Ranger had let somebody else camp in the space I had reserved. He refused to evict them and passed us to a different space. I wasn't happy because we had no shade. Nothing I could do at the time and it was only for one night.