If you're talking about Warcraft, I know for a fact that account management has parental controls built in to it. And I believe other MMOs have similar time-management controls That would require access to the person's account, though, which is against most MMO's EULAs.
The only other thing I could suggest with Vista is to create an administrative account with only you knowing the password, and disable whatever programs you want on the shared account. User Access Control has to be good for something other than irritating users, after all. :)