The users of Mac OS X Lion can disable automatic login using the option provided as part of System Preferences. Click the Apple icon and select System Preferences from the menu list.
In the System Preferences window, navigate to Personal section and click the Security & Privacy option.
This would display the following Security & Privacy window. Click the lock icon available at the bottom of window to make any changes to the security settings.
A popup would be displayed asking to enter the password for allowing changes to the Security & Privacy preferences.
Once the screen in unlocked, mark the check box with label as Disable automatic login. This would disable the auto login for all accounts on Mac OS X Lion.