To disable the user list and guest user login in Ubuntu 16.04 LTS login screen, open a terminal window and do the following:
$ cd /etc/lightdm <enter>
$ sudo mkdir lightdm.conf.d <enter>
$ sudo cd lightdm.conf.d <enter>
$ sudo mousepad 50-config.conf <enter>
Enter the text below into the file.
[SeatDefaults]
greeter-hide-users=true
greeter-show-manual-login=true
allow-guest=false
Save the file.
Reboot the computer. The login screen would not not display the users on the system. It will also deny guest login.
