Hi,

Open your  theme functions.php file and add following code at the bottom:

add_action('wp_footer', function() {
    global $post;
    if (!$post || !isset($post->post_type) || $post->post_type !== 'stec_event') {
        return;
    }
    ?>
    <script type="text/javascript">
        (function ($) {
            $(function () {
                $('select[name="stec_book_date"]').children().filter(':gt(1)').remove();
                $('select[name="stec_book_date"]').children().last().prop('selected', true);
            });
        })(jQuery);
    </script>
    <?php
});