Vodka is actually a much better way to euthanize fish than flushing or freezing. When you flush them they are still alive for awhile, suffering in the hell of your septic lines.
Clove oil is one of the humane ways to euthanize fish, you should be able to get it in the health food section of the store. This puts the fish to sleep in small doses, and kills them in larger ones. So this way you can humanely kill the fish in it's sleep.