How to disable User List on RHEL6 / CentOS6 Login Window?
See http://flakrat.blogspot.se/2012/04/howto-disable-user-list-on-rhel6.html.
Enter (on one line):
# gconftool-2 --direct --config-source=xml:readwrite:/etc/gconf/gconf.xml.defaults --type bool --set /apps/gdm/simple-greeter/disable_user_list true