I want to do the same thing.
I cannot confirm this is what you need to do, but I think you need to check these boxes after logging into your Discourse instance as admin and going to Settings->Login.
The first box looks like it would allow someone to sign on to your Discourse by authenticating through another site. This is not what you want.
The second two boxes are I think what we want.
Maybe someone who knows can verify or correct this advice.
