Do you mean a port excursion/Palo or just something like trivia? For trivia, no one knows you scheduled it but you. I think for Port Excursions or other paid activities you would have to contact Guest Services (or Dining/Port Excursions).
I have cancelled a couple things while onboard. Each time, I just sent a message to guest services thru the app. This was for tastings, and castaway cay rentals.
I tried to cancel an excursion the same way, but had to talk to the excursions manager for that. She called the stateroom and we cancelled that way.
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