Everyone has to go through this at some point. Here are a few alternatives, and additional notes, that may make it easier for you next time. Feel free to add your own.

Euthanasia
Crushing the head with the swift strike of a solid object, such as a hammer.
Placing the fish in water, with an overdose of anaesthetic.
Decapitating the fish with a sharp blade, or scissors.

Previous methods that are no longer regarded as humane, include;
Dropping the fish into ice-cold, or boiling water.
Slow Freezing in the freezer (ice-box).
Snapping the backbone.
Asphixiation, by leaving the fish out of water.
Flushing the fish alive, down the toilet.

Disposal
Incineration is the most effective method of disposal.
Disposal of fish in the rubbish may introduce foreign diseases. They should be wrapped in material thoroughly soaked in a household disinfectant.

Flushing fish down the toilet, or releasing them into natural waters, may introduce diseases into the local environment, and should be avoided.

Remember that some fish diseases can infect humans. Protect any cuts or abrasions, particularly when handling ill fish.

When I ordered my L183s, they had a live arrival guarantee. If there was one that was doa you had to cut the tail off or stick a pin through the head and take a picture. I don't like thinking about doing this to an already dead fish, let alone a live one. I would rather give it to a lfs/lps or somebody that has fish.