I added a link on the comment form to clear the commenter's info.  Some people think it's weird that my site remembers their name and email address.  Don't worry, I probably won't sell your personal information to a marketing company.  Probably. 

I put this PHP code in my header:

if($_GET['deletecookie']) {
   setcookie('comment_author_' . COOKIEHASH, '', time() + 30000000, COOKIEPATH, COOKIE_DOMAIN);
   setcookie('comment_author_email_' . COOKIEHASH, '', time() + 30000000, COOKIEPATH, COOKIE_DOMAIN);
   setcookie('comment_author_url_' . COOKIEHASH, '', time() + 30000000, COOKIEPATH, COOKIE_DOMAIN);
   unset($comment_author, $comment_author_email, $comment_author_url);
   $location = $_SERVER['HTTP_REFERER'];
   wp_redirect($location);
}

And I added a link to ?deletecookie=1 in the comment form.  It's relatively simple, but it took a while to figure out.  I tried a few plugins that claimed to do this for me, but nothing seemed to work, so I went with the manual code. #technology