You may not be able to stop the pending item. However, if your customer claims that the transaction was not authorized because he pulled the plug on the authorization, your customer is entitled to Regulation E's protections. You may have to contact the cable company and demand a refund under Regulation E (since the transaction didn't arrive via the ACH).
Both MasterCard and Visa have services that will effectively block future debits from the cable company as long as that card number is used. There's a fee, of course, but it will help you out for the future. Contact your MasterCard or Visa representative for details.
Your customer has a right under Regulation E to stop these payments, no matter how difficult it may be for you to enforce it. If your customer can document for you that he has ordered the cable company to stop charging his account, you must consider that an "evergreen" stop order for such payments until he lifts it (if he does). Review the
Official Staff Interpretations of section 205.10(c) of Regulation E.