Command line:
Edit LightDM’s configuration File
sudo vim /etc/lightdm/lightdm.conf
Add the following line in the [SeatDefaults] section, save and exit from file.
allow-guest=false
The changes will take effect when you restart your computer.
sudo shutdown -r now
or
sudo reboot
You can also restart LightDM
sudo restart lightdm
GUI:
System Settings > Login Screen(LightDM) > General