Freezing is not humane at all, even though it may *look* peaceful... numerous veterinarians have said so. With cold-blooded animals, ice crystals form in their tissues... this is obviously very painful.
I haven't heard anything about boiling.
If you decapitate, you must also destroy the brain in the process. Even when not attached to the rest of the body, the brains of cold-blooded animals can remain alive for some time.
Do NOT use vodka... it is painful.
Clove oil is definitely the best method. It is used by veterinarians for anesthesia in aquatic animals, and even by humans to relieve pain. Just make sure you use enough... instead of using just a few drops, use 1/4 to 1/2 of the bottle. I've heard of people having bad experiences with clove oil, but it turns out they did it wrong... when doing it the right way, the fish died peacefully.