I agree completely.
If the facts are as reported (and this is a pretty big "if" because we don't know Carnival's side of the story), the family should
bring suit, naming Carnival Corporation, the ship's master, and the cabin steward(ess) as defendants, even though they never intend to cruise with Carnival again. At the very least, they could seek reimbursement of any expenses incurred as a result of being detained, damages for defamation of character, lifting of the ban, and destruction of the paperwork that they signed under duress, and reimbursement of expenses for the lawsuit. It's more likely that the corporate lawyers, when shown the evidence, would agree to a reasonable settlement.