function nwb_remove_search_widget() {
    global $post;
    if ( get_page_template_slug( $post->ID ) == 'searchable.php' ) return;
    unregister_widget('WP_Widget_Search');
}
add_action( 'widgets_init', 'nwb_remove_search_widget' );

add_action( 'the_post', 'nwb_remove_search_widget' );