At present a user can choose whether he will receive an email on new posts or conversations or not. As everything else can be overruled by an admin this should also be possible. Best way for this is imho an option in the user group permisson area which says: "Can receive email notifications". If this is disabled the actual options in the contact details and the preferences ("and receive email notifications of replies" and "Receive email when a new conversation message is received") as well as in the new thread form are not shown (or disabled). Another way could be adding a kind of hook in the email-class to allow addon-coders to realize this in an easy way.