Don't show a particular user account on the login page
Both in 10.7 and 10.6 you will need their shortname then just run this command in Terminal.app

$ sudo defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add user1 user2 user3


To remove them from the hidden user list:

$ sudo defaults delete /Library/Preferences/com.apple.loginwindow HiddenUsersList