The online chat feature can cancel reservations unless they have a dining plan attached. You must cal Member Services. Would have liked to have known that before I wasted the last two hours.
If your checkin is more than 72h out -- you can remove the dining plan from your reservation online, and get the refund shown in the screen there. Then you can cancel the reservation itself through whatever channel you like. No one can remove the dining plan from a cancelled reservation so make sure you cancel your dining plan prior to cancelling any DVC reservation.