How can I unset an option provided by the User Relationships module on the user edit page?
You also don't need to redeclare the $access variable, as you are just removing a field from the form array:

/**
 * Implements hook_form_alter().
 */
function MY_MODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if (($form_id == 'user_register_form' || $form_id == 'user_profile_form') &amp;&amp; $form['#user_category'] == 'account') {
    unset($form['user_relationships_ui_settings']['user_relationship_mailer_send_mail']);
  }
}


That should be all you need.