[_ck_]

This is a tricky problem.

It’s easy to remove the ability entirely to post new topics for all forums (by removing the ability from the “member” role) but to keep a forum visible but not allow a new topic to be created would require intercepting the forum list from the “add new post” sections.

If you can live with not allowing any member to create a new topic you can do this:

https://bbpress.org/forums/topic/only-key-holders-add-topics#post-15339

Then only moderators and above could create topics.

This request comes up at least once a month, someone might try to tackle it someday (but don’t look at me!)