Suche in der wp-includes/comment-templates.php nach dem hier:
 
function get_comment_author_link() {
 /** @todo Only call these functions when they are needed. Include in if... else blocks */
 $url    = get_comment_author_url();
 $author = get_comment_author();
 
 if ( empty( $url ) || 'http://' == $url )
  $return = $author;
 else
  $return = "<a href='$url' rel='external nofollow'>$author</a>";
 return apply_filters('get_comment_author_link', $return);
}
 
und ersetze es einfach hiermit:
 
function get_comment_author_link() {
 /** @todo Only call these functions when they are needed. Include in if... else blocks */
 $url    = get_comment_author_url();
 $author = get_comment_author();
 
 if ( empty( $url ) || 'http://' == $url )
  $return = $author;
 else
  $return = "$author<a href='$url' rel='external nofollow'>@</a>";
 return apply_filters('get_comment_author_link', $return);
}