This came in handy when I needed to disable “guests” from logging into an Ubuntu kiosk I had made. My only administrative access to this machine was via CLI/SSH.

sudo sh -c 'printf "[Seat:*]\nallow-guest=false\n" >/etc/lightdm/lightdm.conf.d/50-no-guest.conf'

The above needs to be in one single line. The below restores the guest login feature – 

sudo rm /etc/lightdm/lightdm.conf.d/50-no-guest.conf