To disable guest account in Lisa Linux Mint 12.

in terminal run command sudo gedit /etc/lightdm/lightdm.conf

add the line allow-guest=false

you can also do it with vi if you prefer.

I had to reboot my system for the change to take affect.

You can go to http://www.linuxbsdos.com/2011/11/26/disable-guest-session-and-create-user-accounts-in-linux-mint-12/ that is were I learned it from.