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.
Also, if you proceed with a card cancellation twice in a row, you will have to wait 7 days before issuing a new card, for security reasons.