Call Delta Sky Miles customer service at 800-323-2323. Have your ticket confirmation number ready to provide details to the customer service agent.
Cancel the flight. Pay $150 per ticket canceled to have your miles redeposited into your account, as of April 2011. Diamond and Platinum Medallion members are exempt from paying fees for canceled miles.
Wait for customer service agent to confirm the transaction has been completed, that the flight has been canceled and your miles have been redeposited.