In my experiments when this glitch was first noticed, I was testing for those who didn't want magic in their game at all, so I wasn't using spellcaster Sims. If you're playing a spellcaster sim and simply not going to the realm of magic, the game may still create other spellcasters (and apparently does.)
In theory, seeding your Other Households with a dozen or so homeless spellcasters in one or two sim households should limit the amount of premades the game pulls to be spellcasters, but I only did a couple of experiments with that. I'm sure you can find some in the gallery (assuming you're playing on PC/Mac)
