Please follow the steps below...
Step 1: Understand that in forum there is no Roles like Subscriber or so. wpForo has Usergroups for this purpose. And each Usergroup is synced with WP User Roles. For example Subscribers are the same users with Registered Usergroup. The Registered Usergroup is the same Subscriber User Role. You can see that they are synced together in Forums > Usergroups admin page. So saying Subscriber Role you should understand Registered Usergroup in wpForo Forum.
Step 2: wpForo forums have permission settings in Edit Forum screen. Edit the forum and use these settings to control Usergroup accesses and privileges in that forum. Each Usergroup has some Forum Access set in each forum. For example The Registered Usergroup ( Subscriber User Role) has Standard Forum Access by default in all forums. You just need to set Read Only forum access for Registered usergroup that forum. Then navigate to Dashboard > Forums > Dashboard and click on [Delete all caches] button.
Step 3: Understand the Forum Accesses. Forum Accesses are set of forum permissions. For example, you just selected "Read Only" access for Registered usergroup. You can find all permissions of Read Only access in Dashboard > Settings > Forum Access > Read Only (edit) screen. You can see that "Can create topic" and "Can post reply" permissions are disabled for this forum access. So your users with Registered Usergroup (Subscribers) will not be able to post create a topic or reply in that forum.
