Yes, a bank can stop ACH payments, but the process varies depending on the specific circumstances.
If you are the recipient of an ACH credit and you believe that the payment is unauthorized or ACH fraud, you can contact your bank and request that they stop the payment.
Your bank will then investigate the transaction and, if necessary, return the funds to the sender.
If you are the sender of an ACH debit and you need to cancel a payment, you can contact your bank and request that they stop the payment.
Depending on the timing of the request, the bank may be able to cancel the payment before it is processed.