IMO using Rules here is not the way to go. You should use a captcha, such as:

##[reCAPTCHA][1]

> Uses the Google reCAPTCHA web service to improve the CAPTCHA system
> and protect email addresses.

[![enter image description here][2]][2]

OR

##[Draggable Captcha][1] 

[![enter image description here][3]][3]

[![enter image description here][4]][4]

Here is an instruction video on how to set it up [Drupal 7 Draggable Captcha - a more friendly way to prevent Spam ][5]


  [1]: https://www.drupal.org/project/recaptcha
  [2]: https://i.sstatic.net/XpIQe.png
  [3]: https://i.sstatic.net/dWof9.png
  [4]: https://i.sstatic.net/kofOa.png
  [5]: https://www.youtube.com/watch?v=rHkiuxZyHCA