Can you stop an ACH transaction from processing?

Generally, from the transaction you will see an option to “void” the payment. It's important to note that o​​nce a transaction has been submitted to the ACH gateway, marking the invoice to stop collecting will not stop the transaction processing. If the transaction returns as successful after it has completed processing, you will need to refund the transaction after it's successfully processed.
https://docs.recurly.com/docs/ach-bank-payments#ach-statuses