If you lose your card you can place a temporary lock on the card until you find it. This will prevent the card from being used.
If you have lost your card for good, then you need to deactivate the card. To do this the card needs to have a balance of $0. Once your balance is $0 please then navigate to your Settings inside the KMS, select your card and select deactivate. You may need to scroll down to see the deactivate option.
Please note that once your card is deactivated you will no longer have access to your transaction history.