This will be implemented in the upcoming version of miniBB. The implementation method is to completely remove a user from email notifications table, when he is set to inactive (i.e. "Member" = No under the profile). It will have no way back to restore subscriptions, but probably it's a good punishment for the user, and a cleaning method for the table.
, you may find the code which says:
add after it:
//remove emails from subscriptions when making the user inactive
if($_GET['activity']==0) db_delete($Ts, 'user_id', '=', $user);
Besides of it, the newer miniBB release will not send email notifications to the users, who have "unconfirmed" status of their email, if it was changed in Profile.