You can remove Siri from the touch bar, though!

I had no idea. The option didn't originally show up in System preferences until I first modified the touch bar preferences via an Apps "view" menu. No idea why this is, but thanks for sending me looking.

If you disable Siri in the System Preferences, it will also not show up in the TouchBar.

I tried this and now if my pinky accidentally grazes past the Siri button with .0001 micrograms of force, it pops up a box asking me if I want to enable Siri.

I guess this is worse.

This one didn't work for me either. Siri had been disabled for quite a while. But, I'm on a work laptop, so perhaps IT caused the problem in the first place.

Go to System Preferences / Extensions / Touch Bar and say "Customize Control Strip...". You will be presented a panel of options for things to drag to your Touch Bar, and your current Touch Bar icons will "wiggle". Use the mouse to drag the Siri icon from the Touch Bar to the panel (yes, this is a thing, try it!) and presto -- you've removed Siri from the Touch Bar.

Yeah. This is so amazingly unintuitive but kind of cool... And it worked!

-edit: In Mojave, I found it in prefs/keyboard/Customize Touch bar