How to forbid acess to a page?

I want to show a "Access denied" (403) page with message "You are not authorized to access this page." for a given route. This will apply to all users in any case.

How to do it in the simplest way?

Just implement a route subscriber like this:

In MYMODULE/src/EventSubscriber/RouteSubscriber.php:

<?php

namespace Drupal\MYMODULE\EventSubscriber;

use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

class RouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  public function alterRoutes(RouteCollection $collection): void {
    // Remove access to page with route "given.route".
    if ($route = $collection->get('given.route')) {
      $route->setRequirement('_access', 'FALSE');
    }
  }
}

Please note the quotes around FALSE!

Of course the route subscriber needs to be declared in the services file of the custom module.

In MYMODULE/services.yml

services:
  MYMODULE.route_subscriber:
    class: Drupal\MYMODULE\EventSubscriber\RouteSubscriber
    tags:
      - { name: event_subscriber }