Drupal - How do I hide the default page /node?

There is no easy way to hide that page. If you don't use it, you can implement hook_menu_alter(), and either remove it completely, or set its access callback to FALSE.

Create a path alias that redirects from /node to the page you want to use as front page.

I prefer to alter the page callback of /node so it shows an error 404 Page.
This always shows your custom 404 page when accessing /node.

function yourmodule_menu_alter(&$items) {
    $items['node']['page callback'] = 'drupal_not_found';      
}