[h1ro]

The tricks that change some variables responsible for the minimum percentage of battery in the upgrade script don't really make a difference here.
After experiencing this problem twice I got angry and just put an "exit" in the first line of the script. Still I get updates and I don't know what triggers it.

How can I prevent updates reliably without cutting network access completely? Is there a special update server?

As an alternative I would consider denying all access and then just whitelisting manually all hosts I want to access with the kindle. Then I could replace the "send to kindle" function in my chrome with something that just uses rsync like on all my other computers. But sadly I couldn't find any good way to format things properly for the kindle. The extension still does the best job and it needs the official kindle email sync to work.