I'm still learning how to use Nabble, but it seems to me that you need to set up User Permissions. You have Unregistered and Registered users accessing your forum.
This is what I did from the outermost Nabble layer so that it applies to all Nabble things...
Select Options > Users > Change Permissions...
There are other options there too, but I haven't used those.
My visitors (hopefully) can read everything, but need to Register if they want a User name, AND if they want to post, they have to 'ask' and if they are from our local area, then I manually put them into the Members group which allows them to post and send them an email letting them know that they can now post.
I haven't set up mailing lists yet, so I'm not sure of their implication, but I think Nabble works the same way across everything, hence its versatility.
I hope this is helpful... otherwise someone else might be more experienced.
Anne