Most of the time this is not possible as Blackbaud does not control certain parts of this process controlled by the bank.
 
If there is a situation where there is an urgent need for this there is a very limited window from when the transaction is made in which Blackbaud can control the transaction. It must at least be in the same day the transaction was made and in some cases, even in the same day, Blackbaud will not be able to stop the processing of the transaction. If there is an urgent need and it is the same day the ACH transaction was made:

Chat with Support and reference this article. 

Include the Event Name and URL, Name and email address of the transaction owner and transaction type (registration, donation, etc.). Also, most importantly, include as closely as possible the time and date the transaction was initiated.