For Ubuntu 16.04 configuration for greeters is taken from the configuration files in the following hierarchy:
The place to add your configuration changes is the second in the list, provided that there is nothing in the last list file to override the values you wish to change.
So, creating a file /etc/lightdm/lightdm.conf.d/50-myconfig.conf and putting in it
- First entry hides the list of login users
- Second entry allows manual user login
- Third entry disables the guest login account
NOTE: For Ubuntu versions prior to 15.10 replace [Seat:*] with [SeatDefaults]
When changes have been made, restart the lightdm service to have the changes applied.
sudo service lightdm restart
sudo systemctl restart lightdm.service
For more information see: Ubuntu LightDM Wiki