It's happened to me a few times at Apex camps (I was soloing).
A group of players just turned up and started killing the small amount of mobs I was targeting. I said "You got to be kidding, right?"... and the reply was "IS THIS YOUR LAND...NOOOO"...
At the end of the day, it boils down to moral conduct, I doubt there are any rules or such being broken.

I just make a mental note of the players to be on the watch out for.