(Credit @MrLemon)
I got them into an unsafe area and lured the zombies to them -- after death they respawn into their own homes.
Alternative methods of murdering NPCs include dropping a bucket of lava over them, and killing them with the Rotten Egg (may require equipping armor and accessories with ranged damage bonus to increase damage done since Rotten Eggs have very low damage)
Rather fun way of solving a problem :)
