Also you can generate a special token (using the visitor's IP for example) and set it as a cookie when your registration form was visited. Then just check it when the form will be submitted. I was using this method long time ago to protect against so-called referrer spoofing. AFAIK this method is still being used by many high-trafficking websites like Pinterest.
Edit: Course ANY protection can be compromised but not by XRumer or regular spambots. The one will need to create a special software to bypass your protection. For example, this my WP plugin allows to automatically pin the post images to various pinboards including pinterest.com, sex.com and many others:
http://www.cyberseo.net/xpinner/
