Ubuntu's login manager, lightdm, offers a guest login option by default.
Most of you won't use it anyway, so why keep it at all? You can disable that entry in the lightdm configuration file, which is /etc/lightdm/lightdm.conf. Open it from a terminal using
sudo gedit /etc/lightdm/lightdm.conf
At the bottom of the file, insert following line:
Afterwards, restart your machine and the guest login option will be gone. To bring it back, just erase the line again or set it to true.