Hi,

Well, you could do it like you've done it now via css display none.

Additionally you could add server side check just in case someone unblock the button.

Place the code in your theme or child-theme functions.php file:

add_action('init', function() {
    function disable_new_organizers() {
        if ('insert_new_organizer' === filter_input(INPUT_POST, 'task')) {
            echo wp_json_encode(array(
                'error'     => 1,
                'error_msg' => 'New organizers are not permitted',
                'data'      => []
            ));
            exit;
        }
    }
    add_action("wp_ajax_nopriv_stec_public_ajax_action", function() {
        disable_new_organizers();
    }, 1);
    add_action("wp_ajax_stec_public_ajax_action", function() {
        disable_new_organizers();
    }, 1);
    
});

This will prevent adding new organizers from the front-end.