Are you talking about removing games they've played from your Played Games list and removing Achievements they've gotten from your Achievements list?
I don't think there's a way to do that, at least not yet. Before I got my own 360, I had a profile of my own on a buddy's machine. He had someone over, and they used my profile to play some games (and got some Achievements). When I eventually transferred my profile over to my 360, those games and Achievements were still on my profile, even though I had never played them, and they can't be deleted or removed. (As far as I know, anyway.)
You might have to create a new profile for yourself, and just let them use the old profile. If you're wanting to earn your own Achievements and start a new Played Games list, that is.