In your theme folder, locate the comments.php and comments-popup.php template files.
Make sure to back up your files before editing, then find the following that starts with and delete it (or comment it out):
<p><input type=”text” name=”url” id=”url” value=”<?php echo $comment_author_url;?>” size=”22″ tabindex=”3″ /><label for=”url”><small>Website</small></label>