There have been at least four lionfish toxins identified: a protein-based toxin (the primary toxin); acetylcholine (a neurotransmitter--slows heart rate); a neuromuscular toxin; and a non-protein toxin. Because the primary toxin is protein-based, application of hot water to the stung area will cause the protein to break down.
I can testify second-hand to the effectiveness of hot water. A friend had a marine aquarium that contained a lionfish. When he was cleaning the tank he accidentally stuck his finger with a lionfish spine. Frantically he called me and I instructed him to go into the bathroom and immerse his hand in the hottest water that he could tolerate, and that I'd be right over. When I got there, about 20 minutes later, it turned out that there was swelling along his finger, but it came to an abrupt stop at one point. Within a week his finger was back to normal. Postscript: About a week later, the lionfish died, prompting all kinds of jokes about who was really toxic.
Rick The only difference between me and a madman is that I am not mad. - Salvador Dalí