I think a random villager will only move in if you set a plot and don't invite anyone from either the island or the campsite. So, the next day, the plot will be "sold" to a random villager.
But if you set a plot and them ask someone from the campsite or island to move in, the next day it will be sold to them.
Spend some Nook Miles and get a Nook Ticket then visit some islands and see if you can look for someone more desirable.
That way at least you won't have a total random moving in.