Another option would be to replace the code Hotel Audit described with the following:
<p><input type="text" name="email" id="email" value="<?php echo $comment_author_email; ?>" size="22" tabindex="2" />
<label for="email"><small>Mail (will not be published)</small></label></p>
What this does is still let the user leave their email address, but it’s just not required.
