Turn off guest session in Ubuntu 12.04

• open terminal
• enter following command  
          
                                   gksu gedit /etc/lightdm/lightdm.conf
  
   
•  add    allow-guest=false to lightdm.conf file
• then restart lightdm using command                                                                                                                     
  
  
                                    sudo restart lightdm
  
  
  Thank You