Remove Zombie Users (WordPress)

<?php
/*
Plugin Name: Clean Up Users
Plugin URI: http://www.dagondesign.com/articles/clean-up-users-plugin-for-wordpress/
Description: Removes registered users that have no posts or comments. Originally by Dagon Design (http://www.dagondesign.com/), the plugin was not maintained for 3.x. Updated by Imperative Ideas. Works up to WordPress 3.8.
Author: Imperative Ideas
Version: 1.1
Author URI: http://imperativeideas.com/


Changelog:
v.1.1 - Updated query to user $wpdb and ensured meta_capabilities key was able to be called by secure sites not using the wp_ prefix.
*/


$ddcuu_version = '1.1';

function ddcuu_add_options_pages() {
    if (function_exists('add_options_page')) {
        add_options_page("Clean Up Users", 'Clean Empty Users', 8, __FILE__, 'ddcuu_options_page');
    }
}

function ddcuu_options_page() {

    // user roles
    $skip_admins = TRUE;
    $skip_editors = TRUE;
    $skip_authors = TRUE;
    $skip_contributors = TRUE;


    global $wpdb, $dddu_version;
    $tp = $wpdb->prefix;


    $result = "";

    if (isset($_POST['info_update'])) {

        // start processing

        ?><div id="message" class="updated fade"><p><strong><?php

                echo "Action Complete - View Results Below";

                ?></strong></p></div><?php


        $result = '';

        $dddu_confirm = (bool)$_POST['dddu_confirm'];



        if ($dddu_confirm) {

            $skip_check = '';
            if ($skip_admins) $skip_check .= " AND LOCATE('administrator', {$tp}usermeta.meta_value) = 0 ";
            if ($skip_editors) $skip_check .= " AND LOCATE('editor', {$tp}usermeta.meta_value) = 0 ";
            if ($skip_authors) $skip_check .= " AND LOCATE('author', {$tp}usermeta.meta_value) = 0 ";
            if ($skip_contributors) $skip_check .= " AND LOCATE('contributor', {$tp}usermeta.meta_value) = 0 ";

            // list of users with no posts and no comments
            $getusers = "
                SELECT $wpdb->users.ID, $wpdb->users.user_login
                FROM $wpdb->users
                LEFT JOIN $wpdb->posts ON ($wpdb->users.ID = $wpdb->posts.post_author)
                LEFT JOIN $wpdb->comments ON ($wpdb->users.ID = $wpdb->comments.user_id)
                LEFT JOIN $wpdb->usermeta ON ($wpdb->users.ID = $wpdb->usermeta.user_id)
                WHERE $wpdb->posts.post_author is NULL
                AND $wpdb->comments.user_id is NULL
                AND $wpdb->usermeta.meta_key = '{$tp}capabilities'
            ";

            $userlist= $wpdb->get_results($getusers, OBJECT);

            foreach ($userlist as $u) {
                wp_delete_user($u->ID);
            }

            $result = 'Users deleted: ' . count($userlist);



        } else {

            $result = 'No option selected!';

        }



        // end processing


    } ?>

    <div class=wrap>

    <h2>Clean Up Users<?php echo $dddu_version; ?></h2>

    <p>A simple plugin that deletes any standard users who do not have comments. For most blogs, these types of users are SPAM. Remember to back up your database before executing this plugin!</p>

    <p style="color: red;">This plugin has been rewritten from scratch with lots of additional options, it is available at <a href="https://github.com/isarmstrong/clean-zombie-users">Github</a></p>

    <?php

    if ($result != "") {
        echo '<div style="border: 1px solid #888888; padding: 5px;">';
        echo '<strong>Results</strong>:<br /> ' . trim($result) . '</div>';
    }

    ?>


    <form enctype="multipart/form-data" method="post" action="<?php echo $_SERVER["REQUEST_URI"]; ?>"  >
        <input type="hidden" name="info_update" id="info_update" value="true" />


        <div style="padding: 0 0 15px 12px;">

            <?php print $formatinfo; ?>
            <h3>Confirmation:</h3>
            <input type="checkbox" name="dddu_confirm" id="dddu_confirm" />
            <label name=""dddu_confirm">I understand that this will permanently delete users from my database</label>
        </div>


        <div class="submit">
            <input type="submit" name="info_update" value="Remove Zombie Users" />
        </div>
    </form>
    </div><?php
}


add_action('admin_menu', 'ddcuu_add_options_pages');

?>