Enable search only with post types:

function mySearchFilter($query) {
    $post_type = 'page';
    if($query->is_main_query()){
        if ($query->is_search) {
        $query->set('post_type', $post_type);
        }
    }
};
add_action('pre_get_posts','mySearchFilter');

function disable_search( $query, $error = true ) {
  if ( is_search() ) {
    $query->is_search = false;
    $query->query_vars[s] = false;
    $query->query[s] = false;
    // to error
    if ( $error == true )
    $query->is_404 = true;
  }
}

add_action( 'parse_query', 'disable_search' );
// add_filter( 'get_search_form', create_function( '$a', "return null;" ) );

// Check deprecated create_function