Cruiserun, it would be nearly impossible to determine how much people had to drink. Yes, they could go from bar to bar and order drinks and the cruise lines could program their computers to instantly tell how many drinks that person had purchased in a certain amount of time-- and, if they were refused service, that would be a sure fire way to start a brawl with drunken passengers but the main problem with your solution is simply this:
How many, and how often, do you see posts asking how to smuggle liquor aboard the cruise ships and every time the question is posted, there are dozens of people ready to answer the question with a " fool proof " way to do so. Therefore, if the cruise lines did cut down on the alcohol served to a drunken or nearly drunk person, what's stopping them from simply drinking their own stash they smuggled aboard?
I would say that a lot of these highly intoxicated people you see roaming around have already been imbibing from their own stash prior to even leaving their cabin. Accordingly a person could be very drunk and the bartender serving him a drink wouldn't even know, as it wouldn't show on the computer, that he / she had purchased any drinks prior to the one just ordered.
It goes back to the same thing most everyone has agreed upon---if a person is going to do something stupid, they surely will find a way to do so. This is one problem that I don't think you can blame on the cruise line. Most people have enough sense to drink and act in a responsible way but there's always that few , no matter whether it's on a cruise ship or the neighborhood bar.