The only way to void them and make it so they can't be picked up by anyone else is to break your void on a full town that is locked down (no one moving in or out anytime soon, like a town that has had all its dreamies for a while and won't be changing)
So you're basically dumping your void onto the town that can't accept any new randoms, but they'll still stick to it as long as its the first town you visit or streetpass with after the villager's house is gone.
Otherwise any villagers with less than 10 or people you streetpass with less than 10 are susceptible to catching your voided villager.