Using Disqus, how to stop storing comments in wp database?

The disqus plugin synchronizes your WordPress comments with the disqus system — that’s it’s main purpose (own your own comments, etc). You can get around this by ditching the built in WordPress commenting system altogether, modifying your theme templates (probably just comments.php) to use the stand-alone disqus javascript.

<script type="text/javascript">
        var disqus_shortname=""; // required: replace example with your forum shortname
        var disqus_identifier="<?php the_guid() ?>"; // globally unique identifier. *should* never change.
        var disqus_url="<?php echo get_permalink(); ?>"; //link-back to this page
        /* * * DON'T EDIT BELOW THIS LINE * * */
        (function () {
            var dsq = document.createElement('script');
            dsq.type="text/javascript";
            dsq.async = true;
            dsq.src="http://" + disqus_shortname + '.disqus.com/embed.js';
            (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq);
        })();
    </script>