unpaid order - blacklist for customers? How do I stop them downloading if not paid?

Hi! Maybe somebody knows how to create blacklist for customers, who didn't send money. My product is downloadable and customer should getting serial number only when I got money on my account.
My idea to automate this process. Create few steps:
1. From the begging all customer in a blacklist.
2. If my bank sen me email, transfer email to DB.
3. Once per day recognize if customer is paid and send email with s/n.
This is idea, but maybe somebody already knows how to ...?
Thank you very much.:lamo: