If you are in a hurry to switch it off, you can always forget the GUI tools and edit the setting manually.
In /etc/lightdm/lightdm.conf remove your username from autologin-user leaving everything else intact:
[Seat:*]
autologin-guest=false
autologin-user=
autologin-user-timeout=0
Re-login (not sure if that is enough - cannot test this myself right now) or reboot.