Вики IT-KB

sudo mkdir /etc/lightdm/lightdm.conf.d

create a “50-no-guest.conf” file under this folder and write the rule allow-guest=false.

sudo sh -c 'printf «[SeatDefaults]\nallow-guest=false\n» > /etc/lightdm/lightdm.conf.d/50-no-guest.conf'

Now restart your computer and the Guest session disappeared.

To revert the changes, just remove the configuration file via command:

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

Источник http://ubuntuhandbook.org/index.php/2016/04/remove-guest-session-ubuntu-16-04/