If you would prefer not to allow Guest Session logins from the Ubuntu Mate 16.04 Login Screen,
Press Ctrl+Alt+T on the keyboard to open the terminal and run the following command:
gksu pluma /usr/share/lightdm/lightdm.conf.d/50-ubuntu-mate.conf
Type in your user password when prompted.
Add this line at the end of file and save it.
allow-guest=false
Reboot to take affect.