Delta really screwed you, all due to the agent's mis-information. They charged you $150 to cancel you final portion of the ticket, which you could have done yourself by not showing up. The $150 charge is specifically for award ticket redeposit/reissue, and Delta's terms clearly state that once award travel has commenced, the entire award is considered used. Not 100% sure but I assume they could have changed the return date over the phone for that $150 fee, if you had a date and flights in mind (also paying the difference in miles).
Definitely follow up with Delta and you should be entitled to the $150 back. I think the miles are lost, though. As stated in the previous post, write a physical letter to mail to Delta management. In the letter, state that the DOT and FTC are being copied as well, and send a duplicate letter to them. That usually gets people's attention. Before you do that, try calling and escalating a little further.
https://www.delta.com/skymiles/about...ules/index.jsp
Unused Award Tickets
Unused Award Tickets may be surrendered to Delta at least 72 hours prior to the original flight departure time for cancellation of the ticket and redeposit of miles into the Member's account. Fees apply. Please visit delta.com/fees for our current fee schedule. Miles redeemed for Award Tickets not cancelled at least 72 hours prior to original flight departure time are nonrefundable. Taxes and fees are refundable upon request. Award Tickets booked within 72 hours of departure are nonrefundable and cannot be redeposited or changed.
Once travel has commenced on an Award Ticket, the Award is considered used. Mileage will not be credited back to the account for the unused portion of the ticket. For information, please visit delta.com or contact Delta directly by calling the number on the back of your SkyMiles membership card or 1-800-323-2323. Unused Pay with Miles tickets are subject to the Pay with Miles terms and conditions outlined below.