Your best way would be to redirect to the theme’s 404 page in the event that the page being loaded matches the slug that you are referring to. Try this:
function check_undesirable_page(){
global $post;
if(is_page() && ($post->post_name=="YOUR-SLUG-HERE")){
global $wp_query;
$wp_query->set_404();
status_header(404);
}
}
add_action( 'wp', 'check_undesirable_page' );
EDIT: Also a good idea to incorporate the is_page()
condition (added above) to avoid undesirable consequences of the page sharing its slug with other items (in case it’s a common word)