Actually, you can't "claim" a spot in general, though it's extremely rude (in my opinion) to just stand on the exact area that someone was grinding at and steal his or her mobs...seriously, what's the point? Is it to "show off" how much better you are? Though it's kind of humorous, when they think they can steal your kills, but they're actually helping you kill.
Sometimes, if I realize that someone is just attacking the mobs that I have attacked first, I just keep at it, they'll give up eventually.
But sometimes, the opposite happens, the person you encounter is actually a better hitter than you, and steals all your kills. I would much rather not
deal with that nonsense, I would have just gone to another area that seemed clear of other players, and if not, change channels.