Unfortunately, it is not possible to delete cards from your Waffle board. This is a limitation within GitHub Issues (and since Waffle is built on top of GitHub Issues, it becomes a limitation for us).
The simplest workaround is either:
- Change the text of your card to fit a new card (re-use the original card)
- Move the card to your "Done" column, open the card modal, and click "Archive". This will visually remove the card from your board.