I am not sure what Dr. Hank is smoking but he is most assuredly incorrect.

Being cold blooded has nothing to do with feeling pain or being able to live in colder temperatures.
The American Veterinary Medicine Association has clear guidelines on how animals should be euthanized humanely. For fish, they should first be anesthetized using something like MS222 or clove oil which is all that the Drs. Foster and Smith product is and can be picked up at any pharmacy or health food store. Once the animal is under and you see no more movement of the gill covers, then it is out. At this point you can freeze the animal before disposal to kill off any infectious diseases they may harbor or to hold it before it can be disposed of.
You should never freeze a live animal, no matter how slowly it is done. The formation of ice crystals in the bodily fluids could be a very real source of pain and discomfort and can not be discounted. When it comes to animals in our aquaria, it is always best to err on the side of caution when it comes to deciding if they can "feel" pain or not.
JCD