There's no built-in parameter for this. You can extend the code to add it.

Add following code in your theme or child-theme functions.php file:

add_action('wp_footer', function() {
    global $post;
    if (!is_a($post, 'WP_Post') || !has_shortcode($post->post_content, 'stachethemes_ec')) {
        return;
    }
    ?>
    <script type="text/javascript">
        (function ($) {
            $(function () {
                if (typeof $.stecExtend === 'undefined') {
                    return;
                }
                $.stecExtend(function (m) {
                    if ('1' === m.glob.options.hide_external_link) {
                        $.each(m.calData.eventsPool, function(){
                            this.link = '';
                        });
                    }
                }, 'onAddToEventsPool');
            });
        })(window.jQuery);
    </script>
    <?php
});

Now in your calendar's shortcode where you want to disable the external link buttons add following attribute:

hide_external_link=1

Example:

[stachethemes_ec hide_external_link=1]

We'll add this text in the next update.

Meanwhile you could add following code in your active theme functions.php file:

add_action('wp_footer', function() {
    global $post;
    if (!is_a($post, 'WP_Post') || !has_shortcode($post->post_content, 'stachethemes_ec')) {
        return;
    }
    ?>
    <script type="text/javascript">
        (function ($) {
            $(function () {
                if (typeof $.stecExtend === 'undefined') {
                    return;
                }
                $.stecExtend(function (m) {
                    if (0 === m.calData.eventsPool.length && m.$instance.$agenda.find('.stec-custom-text-no-events').length === 0) {
                        $('<p class="stec-style-title2 stec-custom-text-no-events">Dates will be published soon</p>')
                                .appendTo(m.$instance.$agenda.find('.stec-layout-agenda-events-all'));
                    }
                }, 'afterFillEvents');
            });
        })(window.jQuery);
    </script>
    <?php
});