Log in to your cruise reservation at the DCL site. Click on the Cruise Activites tab and your itinerary and reservations(excursions, Palo, etc.) you have made should come up. You can then click on whatever day you need to cancel the reservation and you should see everything you have already scheduled. You should then be able to click on it with an option to delete.
I'm doing this from memory, so I hope my memory is right.
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