I finally found some instructions to disable the user list on the gnome login page
http://www.ubuntugeek.com/how-to-remove-hide-users-list-at-login-screen-in-ubuntu-9-10-karmic.html
This was posted in December and I only found it now after several attempts. Google usually does better for me.
Thanks to Ubuntu Geek for posting this info.
I also found a discussion of hiding some users while displaying others here:
http://forums.fedoraforum.org/archive/index.php/t-194030.html