How do you disable Siri permanently on macOS Sierra (10.12.6)?
It is not my work but Rich T. has a great post on this at his blog:

https://derflounder.wordpress.com/2016/09/20/blocking-siri-on-macos-sierra/

You will need to deploy two separate configuration profiles, the relevant keys are below:

Domain: com.apple.assistant.support.plist 

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" 
"http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;Assistant Enabled&lt;/key&gt;
    &lt;false/&gt;
&lt;/dict&gt;
&lt;/plist&gt;


and

Domain: com.apple.Siri.plist

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" 
"http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
    &lt;key&gt;StatusMenuVisible&lt;/key&gt;
    &lt;false/&gt;
    &lt;key&gt;UserHasDeclinedEnable&lt;/key&gt;
    &lt;true/&gt;
&lt;/dict&gt;
&lt;/plist&gt;


Depending on how you manage your macs, and how you want to deploy these profile settings they may need to be constructed differently.

As far as I know these two profiles are the only way to disable this.

Here is a link to the mobileconfig profiles for these settings. You can deploy them with any MDM:

https://github.com/rtrouton/profiles/tree/master/DisableSiri

Otherwise you will need to use defaults write:

defaults write com.apple.Siri StatusMenuVisible -bool false
defaults write com.apple.Siri UserHasDeclinedEnable -bool true
defaults write com.apple.assistant.support 'Assistant Enabled' 0


Once you make these changes you will need to clear the preference cache:

sudo killall -HUP cfprefsd


And then restart the WindowUI server:

sudo killall SystemUIServer


Hope this clarifies.