Is there a way to cancel "automatic tipping" on Carnival cruise before the last day?

When you book a Carnival cruise, and check in, it automatically assigns you the "suggested" amount of gratuities. Despite it being "suggested" and "optional", it is applied automatically by default. And while an in-person opt-out is possible, I couldn't find a way to effectively opt out of them until the last day of the cruise, because:

• Calling their support after booking and asking to remove them doesn't work because "we can't cancel because you are not charged anything yet". Same during check-in;
• Calling it during the cruise does not work, because the service desk cannot (or claims so) cancel the future automatic gratuities, only the accrued ones. Thus you would have to do it again, on the last day;
• Doing this on the last day works, but the line is rather long, as many people try to settle their accounts, pay bills, or ask to change gratuities. The process however is simple and straightforward once you reach the desk - but this could take an hour.

I was told by other passengers that it is possible to opt out of automatic gratuities, but they were either vague about how they did it, or it was one of the above "solutions" which did not work. Does anyone know the solution which works?

PS. Thank you in advance for not starting a discussion about tipping in general.