I admit, I usually make my Sims' lives pretty boring. They're always kind of perfect. So I decided to spice things up and add a little drama. For no reason at all, I had the husband cheat on the wife ...
Yeah, I think the only way to remove a Sim from the household is to use manage household. But you can move them off world instead of to an existing household, which is basically the same thing.
I did this when I had my sim marry a townie only to find out that he was 1 day from becoming an elder...and she was a young adult.
He was older than her parents so I made them get a divorce and then moved him out >.>