I'm not sure why one would want to continue to run UBB.Classic if all of the forum data had been imported into UBB.threads; normally one would utilize the UBB.Classic URL Redirector scripts to send old traffic from UBB.Classic to their new locations in their UBB.threads installation, these can be found in the UBB.Classic Importer archive which is in the member area (in the archive you'd find "ultimatebb.cgi" and "ultimatebb.php"). There is a database table in UBB.threads which holds the locations of all of your old forum ids from UBB.Classic and their mapped counterpart in UBB.threads, the redirector scripts should just send old bookmarked/crawled requests on their way.
The answer to your question however is simple, I don't believe that UBB.Classic had the ability to not allow postings/registrations while keeping the forum open; one could potentially just craft an Apache redirector rule to send requests to the login, registration, new post, and new reply pages to your forum index page on your UBB.threads instance.
The "Board Closed" option in UBB.Classic should close the forums for everyone who is not an administrator.
