Meat tenderizer after a sting is good. There is a product called sea safe that is suppose to help prevent stings (or at least neutralizes them). I bought some for my wife who got stung last year - I have no idea how well it works - we're leaving this Friday. I ordered it online from leisurepro dot com. Below is description from their web site.
Sun Block that also protects against Jellyfish and Sea Lice. The jellyfish stinging cells are activated by several mechanical and chemical events.
The understanding of the biochemistry and cell biology of the stinging organelles enabled the development of both the concept and the technology to inhibit the triggering pathway of the stinging cells.
SafeSea, jelly fish safe sun block, was scientifically designed to disarm jellyfish stinging cells and to protect the skin.