HI!

pls. use following css:

li.menu-item.menu-item-search {
    display: none !important;
}

or

or in the TheGem child theme add next code to functions.php file:

function thegem_child_init() {
    remove_filter('wp_nav_menu_items', 'thegem_menu_item_search', 10);
}
add_action('init', 'thegem_child_init');

HI!

pls. check this plugin:

https://srd.wordpress.org/plugins/disable-search/

Or try to add this code to functions.php of the child theme:

function fb_filter_query( $query, $error = true ) {
if ( is_search() ) {
$query->is_search = false;
$query->query_vars[s] = false;
$query->query[s] = false;
// to error
if ( $error == true )
$query->is_404 = true;
}
}
add_action( 'parse_query', 'fb_filter_query' );
add_filter( 'get_search_form', create_function( '$a', "return null;" ) );