Hm, yeah, this sounds like the feature that comes whit Seasons.
Send your Sim to the Mirror to change appearance, and at the hairstyles, you can see a row of clothing icons. Click the very first so the others get disabled. There you go
Edit: Here's a pic:

You can see the icons on the very right, here they are imaged as inactive.
If you don't want your sims' hairstyles to be changed, make sure your icons look like this.
Image courtesy of
TheBattySoprano.