I have had the exact same problem with Amex (more like Czech porn - just kidding). I could never solve the problem, but I simply check the bill each month. I call Amex and report any charges as being unauthorized and they take care of it. A hassle, I know, but I have never lost any money.

I think simply ignoring the issue could lead to credit rating problems. Also, I think that if you don't question a charge within a certain period of time, you are liable.