Lilith Pleasant was in a favorited household, but she became a witch anyway. I don't think favoring the household might help.
Of course you can play the sims and ask a sage for the Rite of Dissolution, but that's really annoying. To do so, you have to play several households and every time you enter the Magic Realm someone new might be turned into a spellcaster. Seems like a vicious circle to me, but I think it could be a fun challenge for someone who is up to it. ;)
Here are some links adressing this issue:
https://answers.ea.com/t5/General-Discussion-and-Feedback/Townies-in-the-Magic-Realm/m-p/8190436#M15917
https://answers.ea.com/t5/Bug-Reports/NEEDS-INPUT-ROM-Played-Faved-households-becoming-spellcasters-on/m-p/8233049#M105861
Happy simming! :)
