Windows 7: Unauthorized device in homegroup

Hi, all--I hope this isn't a really basic problem I'll be embarrassed about; I set up a homegroup for sharing between my desk top and laptop, both running windows 7 ultimate. A family member recently visited, and even though my homegroup is password protected, with a pretty complicated password, she somehow ended up in my home group even though she doesn't seem to know how she did that. Neither do I! I've been on her computer and she does indeed have access to all the libraries I shared between my own two computers, and I've done all the regular "leave homegroup" steps on her laptop, but still the problem persists. It simply won't leave my homegroup! Any ideas would be greatly appreciated!