How to subscribe/unsubscribe users using rules/rulesets

I would like to build a flexible custom user interface for content notifications using rules. I created two rulesets with a "execute custom PHP" rule to subscribe/unsubscribe a user to/from a content type. Both rulesets accept the variables "user" and a string for "content type".
The "subscribe" ruleset works great, but "unsubscribe" doesn't. Can someone please give me a hint?? Thanks!
The code is largely copy-paste from here: http://drupal.org/node/954754
I think this would be very useful information for many people. There is a notifications_rules module, but it does't work for content type subscriptions.

Ruleset to subscribe users (this works):

// passing two variables to the ruleset: user and a arbitrary string for 'content type'

$subscription = array(
      'uid' => '[user:uid]',
      'type' => 'nodetype',
      'event_type' => 'node',
//      'send_method' => 'mail',
//      'send_interval' => '0',
      'fields' => array('type' => '[node_type_string:string]'),
  );

notifications_save_subscription($subscription);   

Ruleset to unsubscribe from a content type (this doesn't work):

// passing two variables to the ruleset: user and a arbitrary string for 'content type'

// get all node type subscriptions for the user
$subs = notifications_user_get_subscriptions('[user:uid]', 'node', 'type');

// determine if subscription exists
foreach ($subs as $key => $sub) {
  if ($sub->value  == '[string_node_type:string]') {
    $typesubscription = $sub;
    break;
  }
}

//  delete
if ($typesubscription) {
  notifications_subscription_delete($typesubscription->sid);
}