L.B.
Unfortunately b/c you gave them your credit card number the cc company has to continue to make those payments. That is the reason places like this request credit cards b/c they have a signed authorization by you to charge. And unless you jump through the hoops they will continue to charge.
My suggestion to you is to go into the office yourself. People can dismiss letters but it is very hard to dismiss a person causing a scene in your place of business. Obviously don't go overboard but unfortunately in today's world it seems like you have to be mean to get what you want accomplished.
Also as someone else suggested do the letter writing thing and contact the BB as well.
Good luck.