How to change Unity's default login screen to allow users to type in their logins and passwords?
Using LightDM
Edit the the /etc/lightdm/lightdm.conf using this command
gksu gedit /etc/lightdm/lightdm.conf
And add these two lines to the [SeatDefaults] section
greeter-hide-users=true
allow-guest=false
Taken From: http://www.puppychau.com/archives/130