How to completely disable Siri on Sierra?
Ok, I found out the whole procedure by myself, thanks to @blueberryfields and @bret7600.

First of all you need to disable System Integrity Check to be able to edit the com.apple.Siri.plist file.


Reboot in Recovery mode by restarting your Mac and holding Command+R. It can take more time to boot as usual, that's OK, just be patient.
In the top menu go to Utilities and open Terminal.
Type csrutil disable and press Enter.
Type reboot and press Enter to reboot again.


Now you should edit the com.apple.Siri.plist file.


Open Terminal.app
Type sudo plutil -replace Disabled -bool true /System/Library/LaunchAgents/com.apple.Siri.plist


Now you should re-enable System Integrity Check. It is there for a reason.


Reboot in Recovery mode by restarting your Mac and holding Command+R.
In the top menu go to Utilities > Terminal.
Type csrutil enable and press Enter.
Type reboot and press Enter to reboot again.


Now if you check Activity Monitor - you should see no traces of Siri there.
You can repeat this procedure with /System/Library/LaunchAgents/com.apple.assistantd.plist to fully disable it.