This will remove the autoLoginUser key from the loginwindow preference:
defaults delete /Library/Preferences/com.apple.loginwindow autoLoginUser
The password file that holds the user's log-in password is called "kcpassword" and is located in the hidden /etc directory at the root of the hard drive. To remove it, you can run the following command:
rm /etc/kcpassword
You can run these commands via ARD or combine them into a script.