Shutting them out of the US wouldn't change much. It would stop their Alaska cruises, but everything else is mostly non-US countries. It is somewhat common to have fly to some non-US country to get on your cruise already. Those who drive to Texas or Florida to get one will be harmed, but for everyone else they are already flying so where they fly to is just a detail.

The real losses are to the US cities (in Florida, Texas, California, and Puerto Rico) that lose the tourism trade.

Now if other countries like Jamaica get on board things would change fast - but here is a case of each country acting alone loses, only by acting with others do they win. (and there is reward for cheating: if everyone else enforces this and you say you will but don't you get the tourism trade).

Of course if Carnival's customers start caring that will drive changes.