After you have created a contact form with the Contact Form 7 plugin, use this code in your contact form template :

Type in this captcha code :  

[captchac captcha-118 size:l fg:#00ff00 bg:#ffffff]
[captchar captcha-118 4/4]  

[submit "Send"]

And enable the Really Simple CAPTCHA plugin.

To test it, you must log out as admin, or use a different browser.

For more info see the Contact Form 7 website page about this : http://contactform7.com/captcha