Really? How many websites are signed up / do you expect to sign up? Can you not spare 5 minutes per site per month to make sure there's a notification and/or opt-in? Or come up with an automated way to check it. Or use mechanical turk and pay somebody $0.50 per site to check for you.
If you can't prioritize something as important as running an ethical (and law abiding - take a very close look at the ramifications of unauthorized computer usage, which I think it could be argued you're doing with this platform) business, then you really shouldn't be in business.