Cards refer to Ideas, Experiments, Evidence, or Posts.
First, click into the card you want to delete. Next, click the three-line icon at the top and select Delete. A pop-up message will ask if you want to cancel or delete; click the Delete button to confirm.
Remember to invalidate rather than delete your unsupported ideas! You can also use the Status menu to move Ideas you're no longer working on to the Parked, Archived or Completed options once you're done. Once a card is deleted, it cannot be restored.
Looking for information on editing cards? Find it here.