Registered have no permissions to reply
Promotion at 20 posts adds secondary group with permissions.
Most forums I know do something along those lines (usually with moderation for posts instead of no permission to reply - seems to be a good way of weeding out spammers and trolls)
Two usergroups, one can reply and create topics in certain forums, the other (promotion group) can reply and make new threads in whatever forums you want that 20 post limit in. These are all in the node permissions for the usergroups.
[XTR] Post Limit To Create Content This add-on will allow you to set a post count, reaction count or trophy point on a per nodes basis for your members to create new content. This product was created according to the demand by @hasannakyuz...