To remove the guest session, you will need to edit the LightDM configuration file. To do so, open the terminal and open the configuration file by running:
sudo gedit /etc/lightdm/lightdm.conf
The file will read:
Add allow-guest=false to the end of the file so it reads:
Save the configuration file. When you restart your computer, the guest session will be gone.
The only caveat to removing the guest session is that the user menu will be removed from the panel.