keyboard command to reset GNOME

I hate asking stupid questions, but I've got a couple hopefully easy ones maybe someone can solve. Here's the first:
On 6.5 you could easily reset the display manager by using control-alt-backspace at the console. That seems to have been disabled in 7. Although it is sloppy to allow users to leave themselves logged on, it's easier to just log them off using control-alt-bs than to chase after them in an academic environment, where students often leave themselves logged in and the display locked. Although a new user can 'switch user', it incurs the overhead of another login session, and since we are using NFS-mounted home directories, it keeps a mount active and can cause problems when the errant student later logs into another workstation and tries to run such programs as firefox. Although a cron job resets the display manager nightly, by then there can be several GUI logins active on some workstations. It is easier to just log them all off using control-alt-backspace.

Does anyone know if there is a similar capability in 7? (I know control-alt delete is supposed to log you off, but it doesnt work if the display is locked.)

thanks