Disable Guest Session account

My scenario:

Ubuntu 14.04

Tried to disable the Guest Session by editing the /etc/lightdm/lightdm.conf with following value.

allow-guest=false

The lightdm.conf was empty before edit, somehow by writing that caused the screen blank after logged out and wasn't able to login.

So I reboot into recovery mode, and rewrite the lightdm.conf as following:

mount -o rw,remount /
nano /etc/lightdm/lightdm.conf

Insert into lightdm.conf the following:

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

Reboot and the problem solved :-)