disabling html in form textarea

I have a guestbook page and I've added a captcha to it but there's been at least 1 instance where someone took the time to send some spam anyway

the comment was "nice site" with a link - is there a way I can stop someone from putting html in the textarea?

http://us3.php.net/manual/en/function.htmlentities.php

or you can also use strip_tags() (http://php.net/strip_tags)