Stunning or decapitation isn't considered humane unless followed by pithing (destroying the brain). Fish remain alive and conscious for quite a while even after being decapitated.
Ice water baths are considered acceptable for small tropical fish (under two inches). A container of water is placed in a freezer until the surface begins to freeze. Remove any ice and immediately place the fish in it. The temperature shock is believed to render the fish unconscious almost instantly. Gill movement ceases in less than a minute. I can attest to this, I have used the method frequently. (This is not the same as placing a fish in the fridge or freezer, those are considered inhumane.)
Anesthesia overdose is another accepted method. MS222 can be purchased in the USA, I don't know the availability elsewhere.
Clove oil works, but in my experience is irritating and stressful to the fish before losing consciousness. It is not water soluble, keeping the proper dose dispersed and maintained can be tricky.
Garbage disposal euthanasia is indeed probably quick and humane,,,,,,, though it seems undignified and has a major ewww factor.
