If you find yourself with an overly long list of users in the login window or you just want to get rid of that annoying scroll bar down the side, you can stop accounts appearing in the login window.
Start by go to the Accounts pane in System Preferences, and find the “short name” of each user you want to hide. Once you have these, open up Terminal and enter the following:
sudo defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add shortname1 shortname2 shortname3
via 5 Login Window Tips | General, Terminal, Finder | Mac OS X Tips.