You could create a subcategory to restrict access to those topics.
While a topic can only belong to one category, it can have multiple tags. What if the user was allowed to see topics tagged with one but not those tagged with the other? This would make the access rights very complicated and confusing.
To hide topics, muting the tag by default could be sufficient. However, this approach has a limitation: those topics can still be discovered, for example, through search.