Delegate log off privilege?
This replacement GINA should work for XP: http://www.paralint.com/projects/aucun/
Windows 7 I should be able to create an "on idle" task to log off the users after a certain idle time, which will at least unlock the computer eventually.
Had the same problem as you. It seems like a third party application called Unlock Administrator that lets you do just that. You can set exactly which users (even standard users) can unlock a system. There is an option to log off the user instead of just giving them access to the session. There is a separate version of XP and Vista/7