It depends actually on the process how do you see it yourself.
With default miniBB features we could add some new custom field to user's profile which will hold his verification status.
Also, with default miniBB we could achieve that you as admin receive all new registrations notifications on your email address and then you could click on the email link to go to the user's profile. You must be logged in as admin to see, let's say, the link saying "Verify profile". After you click that link, the verification status for the user has been changed.
On the other hand, somewhere under bb_specials.php we need to define new array of forums available only to unverified users. Let's say $unverifiedForums=array(1,2...); etc.
If an unverified user enters any other forum, which is not defined in that array, he will see a warning saying "Sorry, you must be verified to view this forum".
As you see it's quite a big and difficult process to program, so count not less than on $50.
Whatever I could say this definitely is not related to your first question about user IDs...