Adding Checkbox Validation to PHP Registration Form

I'm trying to add a checkbox validation to the form in this PHP file:

<?php?>
<?php get_header(); ?>
<?php if(get_option('users_can_register')) { ?>
<div class="column eightcol">
<script>
    document.getElementById("form-invite").onsubmit = function() {
        return this.rules.checked;
    }
</script>
    <form class="ajax-form formatted-form" id="form-invite" action="<?php echo AJAX_URL; ?>" method="POST">
        <div class="message"></div>
        <div class="column sixcol">
            <div class="field-wrap">
                <input type="text" name="user_login" placeholder="<?php _e('Username', 'lovestory'); ?>" />
            </div>
        </div>
        <div class="column sixcol last">
            <div class="field-wrap">
                <input type="text" name="user_email" placeholder="<?php _e('Email', 'lovestory'); ?>" />
            </div>
        </div>
        <div class="clear"></div>
        <div class="column sixcol">
            <div class="field-wrap">
                <input type="password" name="user_password" placeholder="<?php _e('Password','lovestory'); ?>" />
            </div>
        </div>
        <div class="column sixcol last">
            <div class="field-wrap">
                <input type="password" name="user_password_repeat" placeholder="<?php _e('Repeat Password','lovestory'); ?>" />
            </div>
        </div>      
        <a href="#" class="button submit-button"><?php _e('Register', 'lovestory'); ?></a>
        <div class="loader"></div>
        <input type="hidden" name="user_action" value="register_user" />
        <input type="hidden" class="nonce" value="<?php echo wp_create_nonce(THEMEX_PREFIX.'nonce'); ?>" />
        <input type="hidden" class="action" value="<?php echo THEMEX_PREFIX; ?>update_user" />
    </form>
</div>
<?php } ?>
<div class="column fourcol last">
    <?php if(get_option('users_can_register')) { ?>
    <div class="section-title">
        <h1><?php _e('Sign In', 'lovestory'); ?></h1>
    </div>
    <?php } ?>
    <form class="ajax-form formatted-form" action="<?php echo AJAX_URL; ?>" method="POST">
        <div class="message"></div>
        <div class="field-wrap">
            <input type="text" name="user_login" placeholder="<?php _e('Username', 'lovestory'); ?>" />
        </div>
        <div class="field-wrap">
            <input type="password" name="user_password" placeholder="<?php _e('Password', 'lovestory'); ?>" />
        </div>
        <a href="#" class="button submit-button form-button"><?php _e('Sign In', 'lovestory'); ?></a>
        <div class="loader"></div>
        <input type="hidden" name="user_action" value="login_user" />
        <input type="hidden" class="nonce" value="<?php echo wp_create_nonce(THEMEX_PREFIX.'nonce'); ?>" />
        <input type="hidden" class="action" value="<?php echo THEMEX_PREFIX; ?>update_user" />
    </form>
</div>
<div class="clear"></div>
<?php 
query_posts(array(
    'post_type' => 'page',
    'meta_key' => '_wp_page_template',
    'meta_value' => 'template-register.php'
));

while(have_posts()) {
    the_post();
    echo '<br />';
    the_content();
    break;
}
?>
<?php get_footer(); ?>

I need the submit function to execute only after the checkbox has been checked. So far every time I attempt to add this feature the form goes through even if the checkbox is not checked. Please advise. Thanks.