Turning Siri off on macOS sierra as easy as closing any application. Here are the steps you need to follow to turn Siri off on macOS Sierra.

Go to the Apple menu and choose “System Preferences”.
Open “Siri” from the system preferences options.
Uncheck the box next to “Enable Siri”.
Close System Preferences and you are done. The Siri and its services will be completely turned off until you enable it again.

How to Disable its Keyboard Shortcut:

Similarly you can also disable Siri activation command without turning it off. Choose the "Keyboard Shortcut" option off on the right pane from Siri preferences window under system preferences.

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