See a video of how to do this here.
Freezing Your Spend Card
Go to the Spend Cards module in your left navigation.
You will see options to both freeze the card and report it lost or stolen.
Select the option to freeze your card. Freezing is temporary, so once your card is frozen, you can return to the same section to unfreeze it at any time.
Reporting a Card Lost or Stolen
If you need to report your card as lost or stolen, select that option instead. Reporting a card lost or stolen will permanently cancel the card.