I think there is an option in the admin section to require registration. That means that you have to have an account to view the forums at all. Is that what you are looking for?
If you run IIS, you can disable Allow Anonymous connections and then they would need an NT username and password to get in. But I assume you mean that they must have a Snitz username? Yes?
You would need to check for their snitz cookie on each page of your website to see if they had a username. It is possible, but all of your pages would need to include a bit of code.