add_action( 'widgets_init', 'my_unregister_widgets' );
function my_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_Categories' );
	unregister_widget( 'WP_Widget_Recent_Posts' );
	unregister_widget( 'WP_Widget_Search' );
	unregister_widget( 'WP_Widget_Tag_Cloud' );
	unregister_widget( 'WP_Widget_RSS' );
	unregister_widget( 'WP_Widget_Meta' );
	unregister_widget( 'WP_Widget_Recent_Comments' );
	unregister_widget( 'WP_Nav_Menu_Widget' );
}

Removes all but the text widget, look them up: /wp-includes/default-widgets.php.

Remove WordPress Default Widgets

wordpress

PHP