tavshed

To remove the guest login option you have to modify /etc/lightdm/lightdm.conf.
original
[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu

modified
[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu
allow-guest=false

You can either reboot or restart the service.
sudo restart lightdm