I don't believe there is a way to block the active topic completely.
But you may work around it by blocking access to the forums.
But anyway people not registered or logged in are considered guests.
If you goto
Control Panel » Group Management
to the right of guests are forum,site,cp.
I would start with the site section first.
There are several options there that will limit access to several items.
As far as active topics.
You could goto
Control Panel » Group Management again and select forums.
If you limit guests to not be able to see the forums in question.And or post or reply. That should stop them from seeing about everything except if you have a active topics island they will see the topic but will not be able to open it..
