Well, telling someone to get lost from a piece of no man's land is, in effect, claiming that piece of land. Setting up anything larger than a shelter is, in fact, a kind of claim.
Now, how could this play out?
Well, the NPC might demand compensation for moving, move without (much) argument, threaten you back, agree to a duel according to some set of rules (which the NPC may or may not adhere to), effectively rob you (by demanding you hand over what he wants), demand that YOU leave, or attack outright.
Assuming the NPC did leave, he might come back with backup or ambush you (regardless of which alternative played out in the first part: some people are cheaters, assholes, and/or refuse to accept to lose, which some would consider you're demand from a position of strength to be unfair). This, in turn, can result in attempts to kill the PC, rob him, and drive him away (with the last two not being mutually exclusive).