Hide Siri from default Dock in Sierra
I answered a similar question here: OSX 10.8 - How to keep a custom dock for all users that login

This answer should still apply to macOS Sierra.



Dug up my notes from when I first modified the user template in our base images.  Ran back through the process, changing the default Dock to have about 15 items, on a fresh Mac and these steps worked.  I did a lot of this from the command line, so as to not fuss with changing/reverting permissions using Finder.


Arrange your Dock however you would like for it to appear on new user accounts, changing the orientation and adding/removing applications.
Browse to ~/Library/Preferences and copy the com.apple.Dock.plist file to your desktop.
Open Terminal and start a root session by typing: sudo -s
Enter your current account's password when prompted
Copy the copy of your modified dock preference file from your desktop to the User Template in your Mac's System folder.: cp ~/Desktop/com.apple.Dock.plist /System/Library/User\ Template/English.lproj/Preferences


That should do the trick.  I tested this on a fresh Mac by logging into both a Guest account and a new (created after the above steps) standard user account, both accounts showed the proper Dock with 15~ items.

It shouldn't be necessary with these steps but if you run into trouble you should verify that the /System/Library/User Template folder has permissions set to: System - Read/Write &amp; Everyone - No access.