Although it is a pain, asking the card company to adjust the amount has always worked for me when the retailer was either unable or unwilling to process the payment in the local currency.
The card companies say they take the matter seriously and "can" take action against non-compliant operators. All I can do is continue to bring each case to their attention.
If you have the time, here is a MasterCard DCC Compliance Guide..
http://www.mastercard.com/elearning/...02.17%20EN.pdf