How to cancel service when the company provides almost no contact info

Just as I suspected, my snail mail request to cancel my internet hosting has gone unheeded and I continue to get billing notices by email. They provide only a telephone number and nobody ever answers or returns your call. They also provide a mailing address but naturally mail sent to a mailing address doesn't get a response.

I put in a fake credit card number anticipating such an outcome. I've called them twice asking them to stop billing me. They are either very evil or they are completely on autopilot.

Any information on their legal requirements to me in my efforts to cancel my service would be appreciated.