"Canceling" seems even more problematic than "modifying" on the website too. Several itins I needed to cancel, I would get the "error" screen that asks you to enter everything manually to be sent off, and I don't trust that process to happen before the flight leaves and my ticket is "voided due to a no show". But sometimes I CAN get the modify to work in those cases, and I just change it to some future "holding" flight to preserve the value. Got to love what you have to go through to fly DL these days. (And thank you no change fees or we wouldn't even have that option!)