(The code you look for has changed a bit, but should be easy to find near the top of the file)

Thanks

how would i add a back button to the error message to help commenters try to comment again. as is, just brings up an error page with the message.

also, is it possible to show the message without revealing the location of the wp_comments_post file?

thanks

function wp_prevent_imposters( $commentdata){

// get list of user (display) names for blog
global $wpdb;
$valid_users = (array)$wpdb->get_results(" SELECT display_name, user_email FROM " . $wpdb->prefix . "users");

global $userdata;
get_currentuserinfo();

// get email of current user
$logged_in_email =  $commentdata['comment_author_email'];
$logged_in_name  =  $commentdata['comment_author'];

// see if the comment author matches an existing author
$found_match = FALSE;
foreach ($valid_users as $va) {
  if (trim($va->display_name) != '') {
    if (strtolower($va->display_name) == strtolower($logged_in_name)) {
      $found_match = TRUE;
      break;
    }
  }
  if (trim($va->user_email) != '') {
    if (strtolower($va->user_email) == strtolower($logged_in_email)) {
      $found_match = TRUE;
      break;
    }
  }
}

// if commenter is not logged in, but match was found, block the comment
  if ($found_match == TRUE) {
    wp_die( __('You cannot post using the name or email of a registered author.') );
  }
  else {
    return $commentdata;
  }

}

add_filter('preprocess_comment', 'wp_prevent_imposters');

http://www.saphod.net/2008/10/14/how-to-prevent-commenters-from-using-your-email/

I also tested to see if this modification will work in WordPress 2.5, and indeed it does.

For example, my site uses gravatars but a user could ‘impersonate’ someone else if they know that person’s email, at which point the gravatar will display for a user who may not have made the comment.

Hope this makes sense!