The statement that the warnings that are currently given are not working is to defy the statistics. The fact that we are getting so worked up over one incident is an indication that it is working pretty well. Although untrained people do go into caves, and some of them do die there, the numbers really are pretty small overall. They are working for the overwhelming majority of divers. Only a handful at the far end of the bell curve are ignoring the message.
That does not mean we can't do a better job in some way. Notice that I am not offering that solution. I don't know it.
As I have said before, I don't understand why cave diving is perceived differently from other similar activities where going beyond your training can be fatal. There are no warning signs to be found anywhere. I live in an area with lots of challenging rock climbing sites, and you regularly can watch highly trained climbers on those walls. Occasionally under-trained climbers fall to their deaths. You never then hear a call for closing the sites, providing better warning systems, etc. People kind of figure that if you did a climb for which you are not qualified, then you did something stupid and paid for it.