Restrict messaging - admin -> users or users -> admin only (no user -> user)

Is there a setting that will allow us to make it so that users cannot see the list of other users when using Private Messaging?

Currently, if a user tries to compose a message, he can see a list of all users in the "Send to" field... we want it so that:
1. non-admin users cannot send private messages to other non-admin users
2. non-admin users cannot even see a list of non-admin users in the messaging system
3. admin users can message any user
3a. those users should be able to reply

UPDATE: I just found "capabilities" settings... however, it doesn't seem to be working...
I set it so that subscribers can only send to admins... but,
-- as a "subscriber" I can still see a list of all users (e.g., if I type 'a' in "Send to" I see a list of all users with 'a' in their name... I don't want to allow subscribers to see anyone except admins)
-- as a subscriber I was still able to send a message to another subscriber

Thanks,
Scott