Im surprised no one has mentioned this one.
Take container(ice cream container, whatever is large enough for the fish) and place water in it then into the freezer.
Once a thin layer of ice forms on top of the water, take out, crack ice off and then place the fish in it, its almost instaneous death as the water is getting close to freezing point, which will knock the fish out straight away.
Alot better then putting your fish in the freezer to slowly die....