The safest bet is brain destruction basically ensure their entire head and a portion of the neck is smashed utterly, preferably as fast as possible so blunt trauma from a large mallet would be an idea. This applies to every animal with a brain even cockroaches, remove the head and they don't event know if their hurt they just die. Cockraches starve to death without heads but still die. I have only heard of one animal surviving without its head and that had its cortex left - Mike the headless chicken.
Destroying the brain with a mallet is brutal and might look horrifying but it is an instantaneous humane death. Be aware that the body may still thrash and move, but that is because the brain has been destroyed and the body is spasming. It does not mean the snake is still "alive". With snakes, the heart can even beat for a long time after you destroy or remove the head, but as long as the brain is destroyed, the snake is not alive and is not feeling anything.
Theresa Baker
No Legs and More
Florida, USA
"Stop being a wimpy monkey,; bare some teeth, steal some food and fling poo with the alphas. "
I would say that pithing should only be done after decapitation. The reason being is that people often screw it up. Decapitation results in organ death, pithing then results in brain death.