How to stop a credit card transaction?

I made an online pruchase using my credit card, but have been mis- sold a product and they took more money then was advertised, i have since contacted mbna who told me they cannot stop the transaction and to contact the said company who took the money, i have tried this , but can only reach them by e-mail of which they totally ignore, any advice .
Answer
Call mbna and tell them to reverse the charge as you believe they committed fraud and they can not be contacted.