If you have your confirmation number, you can view and cancel your reservations online. At the top of the Online Dining page, you'll see a "My Reservations" link. Click that, enter your confirmation numbers, and VOILA
I was having a hard time finding WHERE to put my confirmation number ... that site certainly isn't the most user friendly! I thought I might have imagined being able to do that last time. After your post I just kept searching and finally found it under the "My Disney Vacation" spot.
If you have your confirmation number, you can view and cancel your reservations online. At the top of the Online Dining page, you'll see a "My Reservations" link. Click that, enter your confirmation numbers, and VOILA