ExSimGuy,
I would definitely move them, even if they were Royal Platinum Diamond Encrusted frequent Flyers. If you get to explain why in a nice way, people aren't usually too difficult about it. If someone really refused to move, we would have to advise them that the flight would have to be delayed until they have moved (or had to be removed from the flight if they really refused any other alternative). But im my career it never happened that we had to get to this stage.