do you mean villagers that have moved out of her island move in on yours when there's a plot available?
I think that's because the villager goes from her void to your move-in queue (if villagers leave your island without being adopted by anyone they will be in the void and can get picked up by visitors into their move-in queue)
anyways those villagers in your move-in queue will move in if you leave a plot open, only after the move-in queue is empty move ins are truly random
so the easiest thing would probably just be to not let random move ins happen if I'm understanding you correctly