Unregister Default Widget code

function remove_default_widgets() {
	unregister_widget('widget_class_name');
	unregister_widget('another_class_name');
}
add_action( 'widgets_init', 'remove_default_widgets' );

The class names of the all widget is here

WP_Widget_Pages                   = Pages Widget
WP_Widget_Calendar                = Calendar Widget
WP_Widget_Archives                = Archives Widget
WP_Widget_Links                   = Links Widget
WP_Widget_Meta                    = Meta Widget
WP_Widget_Search                  = Search Widget
WP_Widget_Text                    = Text Widget
WP_Widget_Categories              = Categories Widget
WP_Widget_Recent_Posts            = Recent Posts Widget
WP_Widget_Recent_Comments         = Recent Comments Widget
WP_Widget_RSS                     = RSS Widget
WP_Widget_Tag_Cloud               = Tag Cloud Widget
WP_Nav_Menu_Widget                = Menus Widget