Answer:
The Regulation E requirements are found in ยง 1005.10(c), and further explained in commentary to that section. If a consumer directs you to stop payment on a recurring debit card transaction (such a transaction fits the Reg E definition of preauthorized electronic fund transfer) at least three business days before the transaction is scheduled to post, you must accept the stop payment. If you are unable to stop it in advance, you must refund the amount of the transfer to the consumer (if it's a one-transfer stop payment you may also owe the consumer a refund of the stop payment fee).