I've removed the Siri button from the touchbar the day I got my MacBook, more than a year ago, and it never appeared again. Might be a problem on your system? Usually resetting the NVRAM/PRAM solves this kind of problems, but it's basically black magic.
I removed it once. I may be mistaken here, but if I recall, removing it slightly shifted the layout of the other keys throwing off my muscle memory for the f keys.
I'm interested in a solution. I probably hit that key unintentionally 10 times a day.
Perhaps you could replace the Siri button with a similar sized button if one is available, in order to keep the spacing. (I don't have a touchbar Macbook, so I don't really know what's available.)