There are a number of ways to stop a payment that you have already authorized, depending on the type of payment that you have made.
To stop a check, you may be able to cancel it (if you have very recently created the transaction as a mailed check in the app), or you can reach out to support for a check stop payment.
To stop an authorized merchant from withdrawing funds electronically, you will need to place an ACH stop payment to prevent future withdrawls.
To stop an ACH transaction that you have recently sent, you can cancel the transaction up until it has been processed.
For a debit card transaction, you will not be able to cancel the transaction or place a stop payment on the merchant. Once the transaction settles, you can file a dispute in the app.
If you have sent money through a third party (such as a wire sent through Wise or funds sent through CashApp), you will need to reach out directly to those platforms for assistance.