I've used a syringe to inject rubbing alcohol into the fishes head. they died instantly. Anything quick is fine to be honest, the half second or less it takes to kill shouldnt put the fish in any stress.
if you choose to put it down... look up euthanization of fish on the net .. please do not just take it out of the tank.. its cruel.. do not just flush it. below are just a few sites with different ways of putting it down.. .. (for larger fish / rays) i would think a mixture of the clove oil method followed with alcohol methods would be accepted as fast and humane.. read them all on all the sites and fully understand what you are doing as you dont want to do it in a manner that will be unsuccessful, or that will cause undue stress on the animal. that is animal cruelty..
yeah...freezing it horrible...sharp knife to the back of the head is what i always do...quck effective and you know it worked.......use a sharp knife very sharp pute the fish on a plate with a paper towel (prevents slipping) just behind the head/gill plates if its a bigger fish set the knife on it and smack the back of the blade with your hand/mallet....quick done and you know it didnt suffer if you didn't do the right dosage... though the clove oil if done correctly would be a ..les squeamish method...but make sure to do it correctly.