This is actually against DL's Contract of Carriage that you agreed to by purchasing a ticket. What you're doing is considered part of DL's "Prohibited Ticketing Practices". You're using "hidden city" or "point beyond" ticketing with a false destination point to obtain a lower fare.
Instead of refunding you the "unused segment" DL instead has the option to actually charge you MORE. As been stated on this forum, do it a few times and you should be fine. But doing it as many times as you have is just asking for DL to follow through with one of the options below.
In such cases where there is a violation of our Contract of Carriage, we reserve the right to:
- Cancel the remainder of the itinerary and confiscate any unused flight coupons.
- Refuse to board the passenger or check baggage.
- Charge the passenger for the difference between the fare paid and the fare for the passenger's traveled itinerary.
http://www.delta.com/planning_reserv...jsp#prohibited