Webform hook_form_alter and Select Options

I'm trying to make a module for Drupal 7 that replicates the functionality that this custom module provided in D6: http://spinspire.com/article/limited-supply-options-lists-webforms.

The overall goal is to take a webform select options component and modify the available option list based on what's already been 'reserved'. For example, let's say I'm giving away three different laptops for free, and anyone can come and claim each laptop by submitting my webform. When one user claims "Laptop A" (submits a webform with that option), I don't want it to appear as an option in the select list anymore, because it's reserved. Essentially, each option in the select list may only be submitted once.

I took the original code and modified it so it works with D7, up to a point. When I dsm() the option array I see that the previous option has properly been removed. But it still shows up in the form. What gives?

function webform_limited_list_form_alter(&$form, $form_state, $form_id) {

//...
//omitted code to set the option that works just fine
//...
  
  if (!isset($form['#node'])) {
	return;
  }
  $node = $form['#node'];
  if(!empty($node) && $node->type == 'webform') {
	foreach($node->webform['components'] as $component) {
		if(isset($component['extra']['limited_supply'])) {
			_webform_limited_list_remove_used_options($node->nid, $component['cid'], $component['form_key'], &$form);
		}
	}
  }
}

function _webform_limited_list_remove_used_options($nid, $cid, $cid_name, &$form) { 
  $query = "SELECT data FROM {webform_submitted_data} WHERE nid = :nid AND cid = :cid";
  $args = array(
  	':nid' => $nid,
	':cid' => $cid,
  );
  $result = db_query($query,$args);
  foreach($result as $row) {
    $value = $row->data;
	$options = $form['submitted'][$cid_name]['#options'];
	unset($options[$row->data]); //here's where I unset the option
	dsm($options); //this call shows that the option has been removed, but I still see it in the select list
  }
}