I made the mistake of combining households in University after townies had already moved in. When I played the combined household, it says the townies would unclaim their rooms and leave to make room for my new sims. While two of them did so, one wouldn't leave even after I left the household and re-entered it twice.

I did notice that the remaining sim was part of the Secret Society. Is this why he won't leave? If not, why won't he leave and how do I make him leave?

Try the bat box
Move him in so he becomes playable and then move him out
Kill him.

Or less drastically, make him selectable and then make him unclaim his door. Then delete him.

Well yeah, but using a Tommy Gun is more fun. :P

you can just kill him off or have him drop out, easiest way (first make him selectable with the boolprop cheat)

This thread is over 4 months old from the last post if you did not know @vickydeepsecret

omg thanks for that information would have died without it

There's no need to be rude.