A fresh installed Ubuntu 13.04 offers a guest account at the login prompt. That might be handy for others, but I wanted to get rid of it. This can easily be done by adding an appropriate configuration flag to the LightDM configuration at
$ cat /etc/lightdm/lightdm.conf [SeatDefaults] user-session=ubuntu greeter-session=unity-greeter allow-guest=false
After a LightDM restart (or simply a machine restart) the guest account is not offered anymore.