Example of denying access to a Drupal 7 node

permission_node_denied.php

<?php
/**
 * Implements hook_node_access().
 *
 * Runs every time a user tries to view a node on the website.
 */
function my_custom_module_node_access($node, $op, $account) {
  if ($node->type == 'page' && $op == 'view' && !user_access('my custom permission')) {
    return NODE_ACCESS_DENY;
  }
}