Contact Form 7 Spam

I keep getting a lot of spam entries emailed to me. All of them have one thing in common. They all have a spam link in the comments section (and sometimes in another field).

Can someone please tell me how to prevent these spam entries from being submitted by simply not processing the form when there is a link included? I think the easiest way is to just not process the form if it contains “http” in any of the fields.

If someone could provide simple code for this and where to put the code, I would really appreciate. I would prefer not to implement captcha.

Thanks.

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