How to hide or remove unwanted widgets on Multisite installation?

Add this to your functions.php file:

function jpb_unregister_widgets(){
  unregister_widget('WP_Widget_Pages');
  unregister_widget('WP_Widget_Calendar');
  unregister_widget('WP_Widget_Archives');
  unregister_widget('WP_Widget_Links');
  unregister_widget('WP_Widget_Meta');
  unregister_widget('WP_Widget_Search');
  unregister_widget('WP_Widget_Text');
  unregister_widget('WP_Widget_Categories');
  unregister_widget('WP_Widget_Recent_Posts');
  unregister_widget('WP_Widget_Recent_Comments');
  unregister_widget('WP_Widget_RSS');
  unregister_widget('WP_Widget_Tag_Cloud');
  unregister_widget('WP_Nav_Menu_Widget');
}

add_action( 'widgets_init', 'jpb_unregister_widgets' );

This will get rid of all default widgets. If you want to keep a certain widget, remove that line from the function above.