Go to member detail page
Use the cog on the Punchcard plan and select 'Adjust Punches Available'
Set the 'Adjust Punches' dropdown to a negative amount so that punches available will net out to 0.
Done. The person will have no punches left which is essentially the same as cancelling the punchcard.
If you need to refund the member for the charge then scroll to bottom of member detail page where you see 'history'. click on the charge which will take you to receipt page, you can refund the charge from there.