Hide a particular user from the Lightdm login screen in Ubuntu 14.04

In Ubuntu 14.04, to hide a particular user from the LightDM login screen is a little bit tricky because of this bug since 2011:

https://bugs.launchpad.net/ubuntu/+source/accountsservice/+bug/857651

Here is a workaround:

1. Open /etc/lightdm/users.conf:

$ sudo nan /etc/lightdm/users.conf

2. Add the user you want to hide to this line (let's call it trinh):

hidden-users=nobody nobody4 noaccess trinh

3. Change the uid of that user to under 500:

$ sudo usermod -u 499 trinh

4. Reboot