OK, so I see they are entering the security code correctly. However it looks like the security answer is wrong... and I'm looking at your page and that field isn't being validated. Robots actually have a more difficult time answering a stupid question than they do figuring out text in a CAPTCHA. I think we should start by adding validation for that field and see if it helps.
Try adding this line below the validation you already have:
$WAFV_Errors .= WAValidateLE(strtolower((strtolower($_SESSION['random_answer']))) . "",strtolower(((isset($_POST["Security_Answer"]))?$_POST["Security_Answer"]:"")) . "",true,4);
If that doesn't help, then we could try:
1) changing the font in the CAPTCHA
2) adding some content validation as well (ignore if they include http:// or words like "sexy")
3) check the IP address and see if they are coming from a specific range and block those
4) Identifying a different type of captcha that may be more effective like: reCAPTCHA