Actually for those who come across this looking how to disable login screen user list in Jessie the above comment about Debian 8 is incorrect at least as of today. To disable this in Debian 8 Jessie do the following:
$ sudo nano /etc/gdm3/greeter.dconf-defaults
With the file open remove the # in front of the disable-user-list=true line as shown in the file sample below.
# Login manager options
# - Disable user list
# - Disable restart buttons
# - Show a login welcome message
After changing press 'CTRL+X' then 'Y' and press ENTER to save the changes.
After you save the file run the following command to restart GDM and you will be prompted for your login.
$ sudo systemctl restart gdm3.service