To delete/cancel a card, you need to go to the ‘Cards’ section, select the card you want to delete/cancel, and then select 'Cancel card' from the ‘Security Settings’.
Warning! Deleting a card is an irreversible action, i.e. it cannot be revoked.
For security reasons, if you cancel two cards in a row, you'll have to wait 7 days before a new card can be issued.