Unsubscribe users from Simplenews from a specific Newsletter category
I think it should work. Use hook_simplenews_unsubcribe_user which will

/**
 * Invoked if a user is unsubscribed from a newsletter.
 *
 * @param $subscriber
 *   The subscriber object including all subscriptions of this user.
 *
 * @param $subscription
 *   The subscription object for this specific unsubscribe action.
 *
 * @ingroup subscriber
 */
function hook_simplenews_unsubscribe_user($subscriber, $subscription) {

}


Add this in your custom module. I think $mail and $tid will be available in $subscrption variable. 

function module_simplenews_unsubscribe_user($subscriber, $subscription) {
  simplenews_unsubscribe_user($mail, $tid, FALSE, 'website');//$tid -s your genral category tid.
}