Closing the card will permanently deactivate it. If you wish to use the card again with the same details, you'll need to reissue it with the card's details.
You can block or close your card through either the web interface or the app:
Navigate to the Cards section in the left-side menu of the web interface. In the app, simply scroll down to the My Card section.
Select the card you want to deactivate.
Click on Freeze.
Then, click on Close Card. A pop-up for 'Delete Card' will appear; click Delete.