From the Disney site:
If you wish to cancel a confirmed reservation for which you have already applied payment, you must call our Disney Reservation Agents at (407) 939-6244 . Any of the Reservation Agents will be pleased to assist you.
From here at the DIS:
http://www.dreamsunlimitedtravel.com/room_package_terms.htm
Package Cancellation & Change Fees:
No refunds will be given to no shows.
Cancellation: If the reservation is cancelled, amounts paid, minus cancellation fees and other amounts owed, will be refunded. For a cancellation made 44 to 8 days prior to guest arrival, there is a cancellation fee of $100 per package, plus any cancellation fees assessed by hotels or other suppliers. For a cancellation made 7 days or less prior to guest arrival, there is a cancellation fee of $200 per package, plus any cancellation fees assessed by hotels or other suppliers; provided, however, that for a cancellation of a Grand Gathering experience add-on option made 5 days or less prior to the experience date, no refund will be given.
Guest will be responsible for any cancellation fees assessed by an airline. In the case of a non-refundable airline ticket, the cancellation fee is equal to the entire ticket price. Cancellation of a refundable airline ticket must be made at least 24 hours prior to the scheduled airline departure time.
Vacation insurance is nonrefundable. Upon any cancellation, the charge for vacation insurance will be added to the other applicable cancellation fees.