Can I cancel a pending payment (GoCardless)?

A payment that has just been sent for processing i.e. is just pending can be cancelled by following the steps here

Within about 24 hours cancelling becomes impossible even by Gocardless themselves. You must then wait for the payment to clear and refund the customer.

For how to do refunds, see http://support.goteamup.com/knowledgebase/articles/465831-how-to-make-refunds