"In Ubuntu releases starting with 12.04, however, you need to
edit a different file. With the new display manager, lightdm, a
different file is used to manage this setting. So, you edit the
/etc/lightdm/lightdm.conf file instead.
lightdm is an X
display manager that is fast, extensible and provides the ability to use
multiple desktops -- like lubuntu and OpenBox."
When I examined this file on my system, it had my username on the 'autologin-user= ' line, which I tried setting to false like the 'guest-user=false' was already set as. Saved and restarted, good to go now!
The lightdm.conf file,
after your changes, will look something like this:
[SeatDefaults]
autologin-guest=false
autologin-user=false
autologin-user-timeout=0
autologin-session=lightdm-autologin
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