How rude!!! I've had my table hijacked in the buffet, but never the dining room.
If someone has a fixed dining table assigned and someone else just sits there, the waiter should say "Pardon me, but are you sure you're at the right table?" (Well, really, the waiter should say "This is NOT YOUR TABLE. Sit where you're supposed to be." But they have to be polite.) I suppose early on in the cruise, the waiters don't know their assigned customers yet, but supervisors need to tell them to "defend" their tables.