/**
 * Disables the WordPress API.
 */
function disable_wp_json() {
  add_filter( 'rest_authentication_errors', function( $access ){
    return new WP_Error( 'rest_cannot_access', 'Unauthorized access.', array( 'status' => 403 ) );
  } );

  // also remove actions added by wp-includes/default-filters.php
  remove_action( 'wp_head', 'rest_output_link_wp_head' );
  remove_action( 'wp_head', 'wp_oembed_add_discovery_links' );
  remove_action( 'template_redirect', 'rest_output_link_header' );
}
disable_wp_json();