It's easy for an Admin to delete a challenge - past or present. First, select the challenge you wish to delete and open the challenge screen.
Then, tap on the challenge settings gear at the top right of the challenge page:
On the pop-up Settings screen, scroll down to the bottom, and select "Delete":
When prompted, confirm your selection and your challenge will be a thing of the past!
(But, careful: this action can not be undone. Once gone, all data and reports for a deleted challenge are not recoverable.)