Gotta think this is gonna be a tough sell. You have admitted that you are not giving them any revenue these days, and want them to reinstate a liability (to them) that they have already written off. What would make that worth their while? Do you control travel for anyone who does fly on Delta? Can you promise them some flights over the next several months (preferably at full Y fares or close to it)? I gotta think that Delta has to find something in this for them, in order for you to have a chance at persuading them.
If you sent them a change of address and they never changed your address on their account, you may have something. But otherwise, I think you have a difficult row to hoe.
Djlawman