Exclude categories from search query

You can use pre_get_posts action to exclude categories from search query.

function wcs_exclude_category_search( $query ) {
  if ( is_admin() || ! $query->is_main_query() )
    return;

  if ( $query->is_search ) {
    $query->set( 'cat', '-22, -21' );
  }

}
add_action( 'pre_get_posts', 'wcs_exclude_category_search', 1 );

You should paste this code in your theme’s functions.php file.