Communicate With Vendor
Notify the owner of the destination account that an unauthorized fee/payment has been withdrawn from your account automatically and that it must be returned. If it was an overextension of a regularly occurring fee/payment, be clear that the series of payments must be stopped. The method of notification may follow the method of purchase, but other methods may also be pursued. For example, if the payment was initiated on the vendor's website, there may be instructions for canceling online.
Communicate directly to your own banking institution that an unauthorized withdrawal has occurred and must be resolved. Typically, a banking institution will issue some type of temporary credit while an investigation is conducted. The investigation will include communicating with the vendor to determine if the withdrawal was indeed unauthorized. If you have anything from the vendor to prove that the withdrawal was unauthorized, including a receipt or bill for the full amount owed, provide that for your banking institution.
Verify with both your own banking institution and the vendor that the withdrawal has been resolved. Your banking institution should convert the temporary credit to a permanent credit and the vendor should show your account as paid in full. ACH will communicate with both banking institutions to return the funds to your account. Communicate with the vendor to make certain that the vendor's banking institution will no longer transmit a funds request to ACH.
Monitor the withdrawals on your account after the unauthorized withdrawal has been resolved. Monitor the account on the date of the previously regularly scheduled withdrawal date to make certain the it has been canceled and that no further withdrawals from the vendor are initiated.