You can cancel a payment yourself from the Dashboard before 4pm, the day after payment has been submitted.
To do this you simply need to go to the Invoices Page, and click on the actions button per payment, down the right hand side of the dashboard. This button will only appear for payments that are able to be cancelled.
Once a payment has been submitted to Bacs you can no longer 'Cancel' and a refund would have to be processed once the payment has been received by GoCardless.
To find out how, please refer to the support article titles 'Refunding a Payment'