WordPress has many default widgets such as recent post, calendar, meta, pages, rss and others, you can delete all widgets with this function. Add this to your functions.php file

function to Unregister WP Default Widgets

// Source : http://wordpress.stackexchange.com/questions/1567/best-collection-of-code-for-your-functions-php-file
// unregister all default WP Widgets
function unregister_default_wp_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');
}
add_action('widgets_init', 'unregister_default_wp_widgets', 1);