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Not sure if this is feasible, but would you be able to go over and actually collect James every day for a couple of days when you know he's over there?
Say something along the lines of "James has a vet appointment, we need to take him now", "James has to have his worm pill now, so we'll have to take him back home", "James needs his monthly flea treatment", "James needs some expensive dental work"
maybe if you do this a couple of days in a row, they'll get the hint?
Tried that as well- apparently James paddles at his patio door in a way "he can't ignore".