The line actually exists between in-character actions, and out of character actions. Yes, it is perfectly fine, allowable, and even supported (on an RP realm) for a character or group of characters to make a claim to a location. If another player is being disruptive, and it's an 'in-character' reaction to tell the offending character they're not welcome, the roleplayers are well within their rights. It's not griefing. Because heaven forbid one character not like another.
Should it come down to a GM stepping in, they would side with the group, and not the disruptive individual.