function your_pre_user_query($user_search) {
$user = wp_get_current_user();
if ($user->ID!=1) {
global $wpdb;
$user_search->query_where = str_replace('WHERE 1=1',
"WHERE 1=1 AND {$wpdb->users}.ID<>1",$user_search->query_where);
}
}
add_action('pre_user_query','your_pre_user_query');