Since you don't pay for the dining plan until check-in, I would think you could cancel, although you may have to do it prior to check-in . But as a pp mentioned, try posting this on the DVC boards and maybe also call MS....they should be able to give you an answer .
I'm pretty sure that for DVC members you have to cancel the dining plan at least 5 days in advance of your arrival. I do know that you CAN NOT cancel it at check-in.
I did as a pp suggested & posed this question on the DVC board. A moderator told me I have to cancel within 48 hours of check-in, so that gives me a couple of days.