By default, every hub has a discussion feed. To disable posting across the hub, head to the admin pane. Click on the Settings tab and scroll down to General, and click Edit. From there you'll see a Discussion field with a drop-down:
Enabled means participants can post
Locked means existing posts will be displayed but the post field will be hidden so no new messages can be shared
Disabled means all existing posts will be hidden and no new messages can be shared
💡 Tip: It is also possible to apply these settings on a per page basis.