Here's my advice. Find your core processor's report that identifies international debit card transactions. Look at this daily report and see if any are coming from known gambling websites, like PSTARS, Lasvegasusa gambling, etc. If you are getting them, talk to your service provider as they can block gambling websites from getting approval or block the merchant code, or country.
Send the customer a letter as VISA does not allow illegal activity with their cards and since this is illegal, you as the bank are violating Visa regulations. If not already, put in your EFT disclosure that your debit cards can't be used for illegal activity, including online gambling. Other than that, I don't know what else you can do, maybe look at International wires going out to known gambling companies.
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CRCM CAMs