No there isn't any way of preventing them unless you adopt a villager before a random villager moves in (Campsite, adopting from another town, NMT) a random will eventually claim the plot fi you don't fill it
Do you want to keep that plot open for the villager you are looking for using the campsite? you dont need to worry because they will still be able to move in even with your island full. It is just gonna take a while of resitting before the game autosaves until they pick the villager you want gone. I dont know of a way to keep a plot open, sadly.