If you suspect that someone other than you is using your card you should immediately contact your bank and sort out this problem. Legally, the bank cannot obligate you to pay for a transaction that you did not make personally. If the payment was made to an online casino, contact the support team who should be able to provide you with further details. Keep in mind, however, that while you will be able to chargeback the money, you may not have the opportunity to use that card again at the particular online casino. Since credit card fraud is a common crime that businesses suffer from, many will ban the card for further transactions even after only one instance.