Edit the file /Library/Preferences/com.apple.loginwindow.plist. You'll need to be have root privileges, so you'll most likely want to open the terminal and sudo pico /Library/Preferences/com.apple.loginwindow.plist. Add the following two lines right after the <dict> tag:
<key>AlwaysClearUserField</key>When you log out, the user name field in the login window will be blank the next time you go to log in.
<true/>

