By default Ubuntu 14.04 has a ‘Guest Session’ option on the login screen. I don’t like that and also in my tinkering (and for work) I have an extra user account for a service I need to run. I want to hide that option.
To disable the Guest Session, see http://askubuntu.com/a/451537/95754 which has you add a new line to /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
The other fix is from http://askubuntu.com/a/97248/95754 which requires an edit to /etc/lightdm/users.conf
Just add the username you want to hide to the line starting with “hidden-users”.