UBUNTU 10.04 - Hiding users at grafic login screen

I had a problem since I created some additional users for particular tasks but I didnt want they were shown in the login screen.
Edit this file

fem@rodizio:~$ sudo vi /etc/gdm/custom.conf


adding this lines

[greeter]
Exclude=user1,user2,user3,nobody

where user1, user2 and user3 are users to hide at login screen; nobody must be added otherwise it will be shown.
Then reboot..