Move your mouse down, off your screen and INTO the touch bar. This will highlight each menu item. Highlight the Siri button, click, and drag the Siri button upward and out of the Touch Bar. It will now disappear.
You can optionally add a new button to replace the Siri button, but I chose to leave it empty because I inadvertently click in the space all the time.
I always keep an eye on my MacBook's battery percentage—especially since, as a developer, I run a lot of resource-intensive applications.