Open Terminal using the short key, Ctrl + Alt + T
Apply following command to open the configuration file.
sudo gedit /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
Add following line below the last line,
allow-guest=false
Apply following command to open the configuration file.
sudo gedit /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
Add following line below the last line,
allow-guest=false