First, navigate to the Competitors tab. Then, click “Delete all” at the top of the page. Please note: this action is permanent and not recoverable (no control+z here). 

If you can't click the “Delete all” button, it is likely because your competition start time is within an hour. Navigate to the Event Dashboard tab and change the start time to a bit later, and then return to the Competitors tab. You will then be able to see and click the Delete All button. Don’t forget to return the start time to the original time!