First, I'm sorry that you are having difficulty. 

This is actually a common problem, and the more that people do something about it, the better things will be.

Here is a site with a walkthrough on how to do this:  Remove XBox Live credit card

Here is another option once that doesn't work:

If you have asked them to remove the information/stop billing, and they refuse to cooperate (I find that a bit unusual that they would refuse to remove your credit card) this is simple.

Call them again.  Get a name that you called or do it by email.  If you get a refusal notice verbally or written, then write what they said down.

Call your credit card, and state that you dispute the charges (also follow up with a letter after this call) and will no longer be accepting the charges, as you have cancelled the service.  Then write the CEO of Microsoft and give them all the details especially your displeasure.  Send it out.