First, you need to differentiate between these two terms. You can freeze your card, if you don't want to use it for a fixed period of time, or you can block it, if you lost your card or got it stolen.
1. From the main dashboard go to the card section and choose the card, which you would like to block or freeze.
2. First, you will need to freeze the card, by clicking on "Freeze card" button.
3. And only after that, you can also block your card, the button will appear on the same place.
4. You will be also required to choose the reason, why you need to block your card.
5. As last step, you will need to confirm your action by entering the digital code.
Now your card is blocked forever. You won't be able to unblock it. But you will still find the closed card in your card section and the list of all transactions from your card will be available too.