There is 2 ways of deleting cards from a FunRetro board:
You can delete a board if you are logged in with the correct account. You can log in here FunRetro Login And you also need to be the board creator.
Another way of deleting a board is if the board creator enabled the settings to allow anyone to edit the board. It's possible to enable this by clicking on the clockwork icon and after that on "Allow anyone to edit".