I cannot be the only person who will have to defend against this kind of fraud attempt, and so do not know if you are quickly putting together your own Webassist Captcha option for eCart.
That would be my first preference.
If not, a little online research has shown that Google allows one to sign up with them for their 'reCaptcha' service.
You have probably already found it, but here is a link to it, along with a further link showing how to implement it:
I am willing to do this myself if necessary, but do not know what solution you wish to offer.
I have put pertinent information in the private area.