Sounds like the customer Authorized a monthly payment, and now has attempted to inform the company that they are revoking that authorization for future charges.
It is a Reg E dispute and you need to treat is as such & file with your processor using language similar to . . .
The Merchant continues to charge (debit) my account for services I cancelled on _____________________ (date).
Method of cancellation was _________________________________ (i.e. phone, certified letter, in person).
_________________________
My opinions are just that, and might be worth what you paid for them.