To dispatch (euthanise) your crayfish humanely, put it in the freezer for at least 30 minutes. This is the method recommended by the New Zealand Rock Lobster Industry Council.
For best results, cook the dispatched crayfish in boiling seawater. If seawater is not available add a desertspoon of salt to the boiling water.
After the crayfish is cooked (between 6-12 minutes depending on size) drop it straight into a sink full of cold water for 30 seconds to prevent it from continuing to cook. Remove and let drain and cool. Many people prefer crayfish cold or lukewarm rather than hot!