There is no way to disable an Echo remotely unless you deregister it from your Amazon account.
The saved wifi password setting only affects new devices during setup, not devices that you are already using.
Thinking ahead, perhaps you can put the Echo on a smartplug so that you can turn it on/off remotely using the smartplug's app. Dozens of manufacturer's make them, including Amazon, Belkin, TP-Link, etc.