reCAPTCHA has a key system. It makes the user have private and public keys so that no one can simply take your CAPTCHA and use it on their site for others to solve for them. From the API Documentation:
"In order to use reCAPTCHA, you need a public/private API key pair. This key pair helps to prevent an attack where somebody hosts a reCAPTCHA on their website, collects answers from their visitors and submits the answers to your site."
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"Preventing this is virtually impossible."
reCAPTCHA has a key system. It makes the user have private and public keys so that no one can simply take your CAPTCHA and use it on their site for others to solve for them. From the API Documentation:
"In order to use reCAPTCHA, you need a public/private API key pair. This key pair helps to prevent an attack where somebody hosts a reCAPTCHA on their website, collects answers from their visitors and submits the answers to your site."
Unless I understood you wrong, if so, sorry.