In Ubuntu lightdm log-in screen, guest session can be logged in without a password. Although guest gets lower privilege and can’t run
sudo command, you can remove it from login screen via following way.
Edit LightDM configuration file “/etc/lightdm/lightdm.conf”. This need root privilege, use
sudo nautilus command to start file browser with super user. Or run this command in terminal (Ctrl+Alt+T):
sudo gedit /etc/lightdm/lightdm.conf
Add this line and save the file:
Now restart lightdm or reboot to see effect, to restart lightdm execute:
sudo restart lightdm