Well, pretty much what the subject says, is it possible to forbid users from using the "Switch User" feature when the screen is locked by an administrator?
I'm not sure if user switching is a GNOME or X feature.
I personally would recommend not running a GUI as root; it's a quick trip to always running as root (administrator), and that way lie viruses, spyware, and rootkits. They do exist for Linux. (That way also lies accidentally entering 'rm -rf /' on a terminal. As a normal user, it errors out. As root, it erases your hard drive. I learned this the hard way
According to cube3x3 at this forum discussion:
To disable "switch user" in terminal:
When gconf-editor opens, go to 'desktop -> gnome -> lockdown' In that select 'disable_user_switching' .