You could try changing fonts or adding more distortion. I suppose it is possible that a screen reader could read the CAPTCHA. You could try adding the "Simple Question" fields or a honeypot which might be more difficult to get passed. I'm not sure why they would be spamming your form with a robot to begin with. I can't imagine what they would accomplish by doing that in the first place.
How often does it happen? Are they multiple per second? That would imply a robot, or is there always a time gap between sending? That might imply it is a human. You could try to track the IP address and block it, or add validation based on the email address as well.
I've never had a spam issue with sites I've set up with CAPTCHA, but that doesn't mean it isn't possible that you are. It would take research and tests to really determine how or better yet why someone would spam your form.
