You can lock or unlock your card (or employee's card) from the Account or the Cards section.
To lock a card, select a needed card from the list, then open the More options dropdown, and choose Lock Card.
You will get to a confirmation page. If you are sure you selected the right card, click the Yes, lock button.
Once a card is locked, its use will be suspended. This card will be displayed in the Cards section with Locked status.
You can unlock your card following the same steps by choosing Unlock Card in the More options dropdown. You can use your card, once it is unlocked.