How to Disable or Enable Guest Session in Ubuntu 12.04
 

What is Guest Session:-The Ubuntu Guest Session feature provides a convenient way, with a high level of security, to lend your computer to someone else. A guest cannot view the home folders of other users, and by default any saved data or changed settings will be removed/reset at logout. It means that each session starts with a fresh environment, unaffected by what previous guests did.
Want to disable the guest account from showing up on the logon screen? Ubuntu 12.04 comes with a guest account that can be used by guests to access the system.
First open terminal (Ctrl+Alt+T)
and than type this command on the terminal sudo gedit /etc/lightdm/lightdm.conf
now give system password and press enter.
You will get Text editor in that you will see this lines
[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu
now you add this line allow-guest=false
at the bottom of the file like this
[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu
allow-guest=false
now save the file and restart
after restart you will see there is no Guest Session on the logon screen.
Want to Enable Guest Session:- go to Terminal and type this command sudo gedit /etc/lightdm/lightdm.conf
after opening Text editor you will see this lines
[SeatDefaults]
greeter-session=unity-greeter
user-session=ubuntu
allow-guest=false
just remove allow-guest=false line on that file and save that file.
Now restart the system, you will see Guest Session on the logon screen
Enjoy!