If you linked them to your disneyworld.com account (logged in before you made your reservations) then you just need to log in, click on My Reservations > Dining Reservations, and hit 'Cancel' on each one you wish to cancel.
If you did not log in before you made your reservations, then you must log in, then retrieve each one using the confirmation number, and then cancel it after you've retrieved it (they won't be saved and waiting for you).