It doesn't look like it's possible. I've tried removing the app as root, using restricted software and a restriction config profile.

Deterring the user with the other methods you've implemented is probably as close as you'll be able to get.

I created an image where I started the machine in Target Disk mode, the connected to the drive and deleted the Siri app.
Far too much work. I just gave up and worked on preventing the iCloud page from loading

I put both of these in configuration profiles under custom setting and uploaded them one remove the icon and the other disable siri from the computer.

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

and then

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

@mariobarrera You should note what the appropriate Preference Domain is for those profiles

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

I have trouble with deactivate Siri.app, too. I try many things but nothing work fine for me, so i make a policie to delete the app

1. There is a Siri.app under /Application/Siri.app
2. There is a 2nd Siri.app under /System/Library/CoreServices/Siri.app (This is the main app i think, so i delete it!)