this is gonna sound complicated but it will give you an idea of what to look for (or ask)
You can create a new usergroup (USERGROUP B) and posting permissions to YES. While other usergroups (USERGROUP A) you set the posting permissions to NO.
Hm... this makes sense. Only the problem is that I would need to create a new user group just to get that to work. It's kind of inefficient in my opinion and wouldn't work with the user group system that I have setup (though maybe). Thanks anyway.