At every barn I've been to, certain people had "designated" spaces for themselves. To the point if I tied there and they came up, they'd ask me to move. Whhaaat?
I mean, it's rude. Most of the time it's riders who have been at the barn long enough that they "claimed" their spots close the tack room or whatever in a convenient way before there were a lot of riders. For some it's because the space is in front of their horse's stall, which is more understandable than just "I like this post, it's close to the tack room and I'm lazy so MOVE!"
That being said, I think it is incredibly rude of this person to do that, and if I may say so, a bit passive aggressive. I would talk to the BO if I were you and say "This person came up and did this the other day and I was a little shocked...are there some posts that are typically only used by certain riders? I would appreciate it if said person didn't risk the safety of the horses to prove a point in the future though."
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers