You would need to edit your template files to get rid of all the places that tags show up. If you are using the default theme, the best thing would be to edit all the files that need editing and upload them to a new folder /my-templates/themename/ as that will allow you to edit the files that you need without doing anything to the themes in /bb-templates/. Any file in the folder inside /my-templates/ will be read and if it doesn’t exist there, it uses the originals in /bb-templates/.
Off thet top of my head, it would require editing the following files:
forum.php (to get rid of the place to create topic and tags)
There might be more and I might be off on the names, but just comment out <! — and –> around any code that deals with the tags.