If you want to keep a user account on a (Yosemite) Mac but not advertise it on the login screen, you can do so by entering this command into the terminal:
sudo dscl . create /Users/username IsHidden 1
where username is the username of the account you're trying to hide from the login screen. Even though you see the word create in the command, you should actually run this command on a user account that already exists—you're creating a parameter that says it's hidden, not actually creating a user account with this command.
Note: this does not make the account more secure. All it does is simply not advertise the account on the welcome screen (and in System Preferences > Users & Groups). Users can still easily find out all the usernames on the computer by (even with a non-admin account) typing
ls /Users
into the terminal.
To unhide the user, simply run the same command but change the 1 (yes) to a 0 (no).
Instructions thanks to How to Hide a Specific User Account from Login Screens of OS X.
For more details on how you can do this in Mavericks and previous version of OS X, check out How to hide a user account in OS X.