Hide User

If your system uses AccountsService, you cannot hide a user from the greeter screen by reconfiguring lightdm because it defers to AccountsService. That is stated very clearly in the comments in:

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/etc/lightdm/users.conf

What you need to do instead is to reconfigure AccountsService. To hide a user named XXX, create a file named

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/var/lib/AccountsService/users/XXX

containing two lines:

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[User]
SystemAccount=true

Reference

http://askubuntu.com/questions/92349/how-do-i-hide-a-particular-user-from-the-login-screen

Thanks to

Yu Zhang, who is my senior fellow apprentice.