Why won't an NPC go into his house?

If your demolitionist has claimed a room offscreen, you may need to go far enough away that he teleports 'home'. He's not going anywhere because he can't reach his house in the first place.

Kill demolitionist with lava. Then he will respawn after some time. And maybe he will choose more desirable place :-)

When NPC begin to live in house not suited for him, I kill NPC and destroy chair and table. And in next time NPC chooses another house :-) After that I place taken off object on their place.

P.S. Magma is a solution for every problem :-)