[livibetter]

Have you tried to search one in Extend?

If you find no plugin and you can do code, I think you can check line 18 in /bb-post.php, line 479 in /bb-includes/capabilities.php.

You need to check current user’s is allowed to start a new topic in specific forum or not in your filter hook.

Just thoughts, but should work.

PS. I think it’s better not to do too much that bbPress can’t by default, or you could suffer more after upgrading unless you completely know what you do.