To modify the existing page you can edit file stachethemes_event_calendar\view\front\pages\single.php

To create one from scratch, in your theme main folder create file single-stec_event.php.

Try adding following filter in your theme or child theme (whichever is active) functions.php file:

// Disable active sidebar for stec single page
add_action('stec_single_wp_head', function(){
    add_filter('is_active_sidebar', function(){
       return false; 
    });
});