Is it possible to block a lost card?

This isn’t supported. Once a backup has been created, all cards in the set have a single private key and become equal, so there is no technical way to identify which card has been lost.

When you activate a card and create a backup, you protect each card with its user password. Moreover, the card is protected against brute-force attacks. After the sixth incorrect attempt to enter the code, the delay time for the next attempt is increased by 1 second. The maximum delay time is 45 seconds. The delay is only reset after the access code has been successfully entered.