Yes, you can cancell online. Retrieve the reservation using the confirmation number and the phone number that you used to book the reservation. Once it's retrieved, there will be an option to cancell.
You should be able to log in to disney's website. Go to my vacation. then my dinning reservations. Put each dining reservation and the phone number you used when you made them. It will pull up the reservation and you can click cancel.
or you could just call Disney Dinnig and cancel.
I had to cancel some for June- just couldnt get the dates worked out.
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