That's the merchant's problem. If he can't cancel the transaction, then he could for example pay you a cash amount to compensate you for the fact that he used DCC against your will, or issue a partial refund to your card.

Keep in mind that Visa's rules state that the merchant can't place any restrictions on non-DCC transactions which don't also apply to DCC transactions. For example, if a customer who chooses to use DCC simply has to sign a receipt and after that can leave the shop, then the same option must also be offered to a customer who chooses not to use DCC. If your choice not to use DCC comes with extra restrictions (such as requiring you to argue with the shopkeeper for half an hour or requesting a chargeback), then the merchant should pay you compensation for the extra restrictions.