Here is the code:
under /wp-content/themes/your_theme/sidebar.php
You will find a div with the next code:
<div id="secondary" class="widget-area" role="complementary">
<?php dynamic_sidebar( 'sidebar-1' ); ?>
</div>
Just comment (or remove ) the line like this:
<div id="secondary" class="widget-area" role="complementary">
<?php //dynamic_sidebar( 'sidebar-1' ); ?>
</div>
Also, there are Plugins that disable comments for all Post or Pages. Like this one: http://wordpress.org/plugins/disable-comments/