[Plugin: Contact Form 7] restrict html code in message field area?

Using contact form 7
How can I restrict people from putting html code into the “message field”, but allow html code in the email that comes to me?

I’m sure there is something I can add to “Default value” or the “class” but I can’t think of what?
Suggestions?

http://wordpress.org/extend/plugins/contact-form-7/