I hesitate to jump in here since this issue comes up every couple of months, but it is probably worth another comment. Blue-rings, specifically species in the genus Hapalochlaena, contain TTX in their saliva. This is the same compound found in pufferfish that is responsible for Fugu poisioning. TTX blocks neural conduction and is lethal to humans, usually causing death via suffication. It is well documented that the bite of the two south Australian species, H. fasciata and H. maculata, can kill people. I don't know of any data showing that the bite of H. lunulata is deadly, but reports of blue-ring caused deaths in areas where H. lunulata occurs suggest that it is. Personally, I have experienced some neurological symptoms just handling specimens, so I'm convinced that being bitten by one would not in your best interest.
That said, telling people simply how deadly these animals are is not going to convince them that they should try a different animals for their tank. In fact, it usually has the opposite effect. What should convince them to look elsewhere is how difficult these animals are to keep and how quickly they die. H. lunulata are almost alwasys sold in the U.S. as adults. This means that they will live at most three months and most not that long. If the female lays eggs, she will move into a cavity and brood for a month, but you won't see her for that time, then she dies. Many adults never adjust to captivity and begin eating their arms. Others pace the walls of the tank until they die. It is the unusual animal that behavies "normally" in even a fairly large tank. Given the cost of the animals, how short a time you will have them, and the impact on the species that promoting their collection is causing, I really can't recommend purchasing one.
Roy