In the mobile app, just tap on the card you would like to freeze and then "Freeze card".
In the web app: Sign in and head to the 'Cards' section, select the card you would like to freeze and in its setting section click ‘Freeze card'.
When a card is frozen, no payments can be made, which means the card is made temporarily inactive. You can also mark cards as “Stolen” or “Lost” after freezing them, in which case your administrator will have to order a new card for you. Surely, you may find your card later, then please contact our support team and we should be able to reactivate your previously lost card.
Be aware that only administrators can unfreeze cards. If you need to unfreeze a card, please contact your account administrator.
Related article: Why has my card payment been declined?