Under Ubuntu 11.10, the login screen provides users with the possibility to logon to a guest session, which is a new feature in the new LightDM display manager. The guest session allows any user to login to Ubuntu without a password, but all changes made by this user will be discarded once he/she log out the guest session.
There are some users who see this functionality as useless and want to get rid of it, but it's not really a recommended step because once removed, you will also lose your profile indicator displayed in the top panel:
1. Open the terminal under Ubuntu 11.10 and edit the lightdm.conf file with this command:
sudo gedit /etc/lightdm/lightdm.conf
2. At the end of the file, add this line:
It will be something like this:
Save your file and exit.
3. Restart now your system. The guest session will not appear now in the login screen:
4. To restore the guest session as well as your profile indicator, remove the "allow-guest=false" line from the /etc/lightdm/lightdm.conf file and reboot your computer.