I have used the freezer method and this is why... I have read that as the water gets cooler and cooler, they start to go to sleep. Eventually it gets so cold they don't wake up again.
Allegedly it is a peaceful way to go. I guess chopping the head off would work too but I consider my fish pets and could never bring myself to do that.
By flushing a live fish you are subjecting it to all kinds of inspeakable things. There is no euthanasia involved, they die a horrible death by being poisoned and suffocating. Please don't do it!
I have also heard that you can euthanize a fish by using Alka-Seltzer but I don't know enough about the administration of this to comment.
Katie