If you want to restrict users from appearing on lightdm login list, proceed as shown below :
1 – Edit the file /etc/lightdm/users.conf with root privileges.
$ sudo vi /etc/lightdm/users.conf
2 – Add the user name that you don’t need to be shown in the line hidden-users=nobody nobody4 noaccess oracle
3 – Save. Next time you log in you will not see that user in the login list.
PS. Users having shell = /bin/false e /usr/sbin/nologin will not be listed by default.