The registration page has a built in reCaptcha field and you just need to input your keys in the ClassiPress admin to set it up (see the
ClassiPress admin demo > Settings > "Security" tab. The sidebar contact form in the ad listings doesn't use reCaptcha, but a simple captcha in the form of a math question. Some customers have modified the theme to include the reCaptcha on this page too though, so it can be done with some customization.