To remove the guest session from Ubuntu logon screen

Open the terminal (Press Ctrl – Alt – T) and run the following commands.

sudo /usr/lib/lightdm/lightdm-set-defaults -l false

To undo it or re-enable the guest account, run the commands below.

sudo /usr/lib/lightdm/lightdm-set-defaults -l true

Or run the commands below to open lightdm.conf file

gksudo gedit /etc/lightdm/lightdm.conf

Then change the line highlighted below to false and save the file.

allow-guest=false