Not really possible at the moment. Every user is a wp subscriber.
Shucks. Any plugins that may solve this?
you should implement or extend the roles of wordpress to work with, another solution is to use buddypress profile field’s and BuddyPress xProfiles ACL plugin to define roles
You can limit access to forums by putting them behind private groups. By default, your members have the ability to post on the activity stream, but limiting access to groups, either by setting the Buddypress Admin panel to only allow Admins to create groups, or by using the Limit Group Creation plugin, you can keep your users out of certain forums.
Currently there is no out-of-the-box way to allow read-only access to a buddypress install; but given that it’s a SOCIAL network, it isn’t really a logical functionality to have: seems anti-social.