Simply remove the "Create_topic" and "Reply" permissions for ALL groups in your parent/root application.
All your sub-forums will then inherit these permissions, so in each and every sub-forum, go to the permissions screen and add back those permissions for the groups you want to be able to post.
For example, you might want to have all sub-forums disallowing "Anyone" from posting, while on some others you can add those permissions for "registered" users, and if you have some premium or special customers, that you have placed in a "Members" group, then still other sub-forums would give them those permissions while still not allowing "Registered" users to post or reply.
Note sub-forums always inherit the permissions set for their parent unless you over-ride them with more local permissions.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.