It may be possible; I would certainly try. But you will have to sue them in the small claims court. You would represent yourself, the fee is small ($30 or so), and it becomes part of judgment if you win the case.
What you could argue is that the "we charge you automatic gratuities which you have to jump through multiple hoops to cancel" was not part of the original terms in the cruise purchase. I've never seen it presented clearly while buying a cruise - especially if you buy it at 3rd party retailer like Costco travel. And this is an important term.
Those terms are introduced later, during the check-in procedure, as part of "cruise contract". However at this moment the cruise has been already paid, and you're not having an option to decline the contract and get the full refund. Thus what Carnival is essentially doing is coercing you into contract modification, adding new terms which were not disclosed during the purchase. And the terms itself are not very clear.
Carnival also knows that this practice is shady, considering how deep they hide it. Thus once they are served with lawsuit, they will likely try to settle it with you. Don't waste your time talking to their support, they can't do anything. Like with most American companies, the only people who have real power in the company are those who read the court papers.