By default ubuntu 12.04 comes with guest account.You can disable this account using the following procedure.Guest account is a paswordless account which allow users to get access to Ubuntu machine
Open /etc/lightdm/lightdm.conf file from your terminal using the following command

gksudo gedit /etc/lightdm/lightdm.conf

Add the following line


Save and exit the fileAfter adding the above line you should see similar to the following in lightdm.conf file


Finally you have to restart lightdm using the following command from your terminal

sudo restart lightdm

Note:- After executing above command all graphical programs running will be closed