Ubuntu login page: where to specify users list shown

In /etc/gdm/custom.conf, add the following stanza:

[greeter]
Exclude=nobody,wont_appear1,wont_appear2

If you want an inclusive list, I think the following will work (though it will also show users who recently logged in by entering their name manually):

[greeter]
IncludeAll=false
Include=will_appear1,will_appear2

Users with a UID <1000 won't appear unless explicitly included (I don't this this is configurable). Any user can still log in by explicitly typing their user name.

Reference: gdm manual

If you want to forbid some users from logging in (as opposed to not showing their name and picture), add a filter to /etc/pam.d/gdm. Something like (untested; see man pam_succeed_if for details):

auth  required  pam_succeed_if.so user ingroup may_log_in_with_gdm

If you want to hide entire list open the terminal and enter the following comamnd :

sudo gconftool-2 --direct --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory --type Boolean --set /apps/gdm/simple-greeter/disable_user_list True

For remove only some user for the list you need to change UID to a number < 1000

The gdm login list lists any Ubuntu login user with a UID >=1000

This means as long as you get a login set to use a UID <1000 it will NOT show up in the gdm login list.

here's how:

System -> Administration -> Users and Groups

Edit the user by click on properties, go to advanced, then change the "User ID" to a different number. Try and make sure it's not already used by another person by looking at /etc/passwd