Throw it really hard onto the ground...otherwise known as blunt trauma. Did that to a suffering goldfish once, wasnt pretty. Bleach would kill it instantly too.
I used to think bleach would kill a fish instantly, but turns out it wasn't the case for me. I had a silver dollar once that got sort of crushed as I was rearranging some rock work in the tank. It was obviously not going to survive as the whole rear side of the fish was nearly torn off. I quickly got a small cup of bleach and dropped him in, It was horrific.
The fish started twitching and spazzing out, then blood started oozing out from the gills... It was bad. Took a good 2 minutes for it to stop moving.