I would call or see a vet. They have a reptile sedative.

I have heard of people refridgerating the snake to slow it down thinking it will sedate it, then decapitating it. Not sure how the snake feels.

Do a search on brain pithing (spelling?). It is where you drive a spike (nail) into the snake's brain. Supposedly the most humane way even though it sounds like the worst. Since snakes have pea brains, you would have to know where. A vet could be the best source of info.

Uncomfortable subject, but it is something we all should be aware of. I am going to the vet with my dog next week, and I will ask them about this subject.