Cards were recently moved into their own tab in the sidebar. If you click on that tab you can then select 'hide' on the deck you want to hide. However, if you want to delete a deck you can double-click on the name of the deck and then scroll down to the bottom where the delete button is located. Note: you will not be able to get the deck back. Delete is permanent. Generally, I recommend you hide the deck instead of deleting it. - Gauss