Sometimes you may want a little privacy even from your smart devices. Whatever the reason, you can deactivate the "Hey Siri" voice command on your HomePod so it's not always listening.
To disable the "Hey Siri" option on your HomePod navigate to the settings in the Home app on your iOS device, then tap the "Hey Siri" toggle so it is OFF. You can also simply just say "Hey Siri, disable Hey Siri." The HomePod will then ask you to confirm with a "yes" to disable the voice command feature.
You will need to use the HomePod settings turn back on the "Hey Siri" toggle or use the HomePod touch controls to use Siri again.