If you want to keep widgets out of client’s hands you can use this snippet:

function unregister_default_wp_widgets() { 
unregister_widget('WP_Widget_Calendar'); 
unregister_widget('WP_Widget_Search'); 
unregister_widget('WP_Widget_Recent_Comments'); 
} 
add_action('widgets_init', 'unregister_default_wp_widgets', 1);