Can I create a keyword filter on emails sent from an email form with Rules? Can I make a Rule that will check content of an email sent from a contact form and block the send if a condition is met?

I was hoping I could set up a Rule to react on event Before Email Send, or something of that kind and then check the content, but that event doesn't exist: could I create it? Or is there a better way? 

Context: I have a content type where each node is an advert for a local attraction. I have an email field which is set to display as an email contact form which can be submitted by anonymous users.  I am using Mail System and Mandrill to send the emails. 

Although I have Spamicide module and Captcha already installed, the odd spam email is creeping through the form.  The spams are usually highly obvious and would be simple to eliminate if I could set up a keyword blacklist, but I am not sure how best to create one.
Using hook_mail_alter https://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_mail_alter/7 you can check/change all mails before they are being sent.

Setting $message->send = FALSE; will stop the mail system from sending out the mail.