Change the permissions for the "General" group. You need to change it from "Allow All" and highlight the Restricted Forums/Categories to NOT be allowed into.
And to keep the Anonymous users from seeing any Categories/Forums, you would need to set the Categories/Forums as restricted and not allow the General Group access to them.
I would suggest having a "viewable" area for visitors to have at least view access to so you have a place to put instructions on how to sign up, rules, etc. You can also set this Forum to not allow anonymous posts or replies to keep it "clean." [originally posted on jforum.net by GatorBait3]