I tore through this issue over the weekend and I am happy to say that I have resolved the NCA issue on your site. What was happening, you ask?? Well, I am not entirely sure. However, this is what I do know.
1. Your Public user group had 1 user assigned to it. I noticed on my test sites that no users are assigned to Public. Now, I was not able to locate this user on your site. But, I could see them in the user group tally (Users>>User Groups).
2. When a visitor is on the site and is not logged in, they have a user id of 0 (zero). When the extension looks up the user's user groups it passes the user id to the database. Now normally for a Public visitor with an id of 0, the database would say, "Hey, wait a sec, there is no user with and id of 0". So, then the extension would build a false record for the visitor. This false record then tells the extension that the user is not allowed to see the premium content.
However, on your site, when the extension looks up the Public visitor, the database finds a record and tells the extension that the user id 0 belongs to your two premium user groups. What, what??
I didn't have database access for your site, so, I didn't explore the mystery "Public" user. Instead I simply told the extension to not even bother looking up the user in the database if their id is 0. Now, every thing seems to work!
I am creating a new version of the extension 1,04. It will contain the changes I made for your site. So, when you upgrade, you won't have to worry about the changes reverting back.