Redirect all search related trafic to a 404 page.
<?php
function wpb_filter_query( $query, $error = true ) {
if ( is_search() ):
$query->is_search = false;
$query->query_vars[s] = false;
$query->query[s] = false;
if ( $error == true )
$query->is_404 = true;
endif;
}
add_action( 'parse_query', 'wpb_filter_query' );
?>