Unregister Genesis default widgets. Source: http://my.studiopress.com/snippets/admin-management/#unregister-genesis-widgets

unregister-genesis-default-widgets.php

<?php

//* Unregister Genesis widgets
add_action('widgets_init', 'prefix_unregister_genesis_widgets', 20);
function prefix_unregister_genesis_widgets() {
    unregister_widget('Genesis_eNews_Updates');
    unregister_widget('Genesis_Featured_Page');
    unregister_widget('Genesis_Featured_Post');
    unregister_widget('Genesis_Latest_Tweets_Widget');
    unregister_widget('Genesis_Menu_Pages_Widget');
    unregister_widget('Genesis_User_Profile_Widget');
    unregister_widget('Genesis_Widget_Menu_Categories');
}