actually, the receiver of the credit - your customer - has a right to refuse the credit and your return reason code would be R23 - Credit Refused by Reciever. There are 6 reasons listed under this return code, the last being "the receiver has not authorized this credit entry to this account."
Ideally, she will tell the originator to stop the credit, but this method is also allowed per the NACHA Operating Rules.
_________________________
Providing alternative truths since the invention of time