Go to Apple Menu-->System Preferences-->Personal-->Login-->Login Window tab Where it says "Display Login Window as..." select "Name and password entry fields". When you boot up, no users will be listed and you must enter the Username and password for the user you want to login as.
*NOTE: For some reason the login window remembers the last user that logs into the machine, so the username of the last logged in user will be displayed. The password field will be blank, though.
Hope that helps.