There is a tool that renames the OTA binaries. But there were reported instances in the past, where updates would still get installed.

Its a bit murky, because you can find conflicting info about this.

I could (a lot of users might not have such an option in their router), but that would mean I wouldn’t be able to use some of the features of koreader, either. I could blacklist the urls individually, but I don’t know if there is a list of relevant urls

My kindle has long been jailbroken, unfortunately there doesn’t seem to be a way to completely de-amazon it. The device will still always boot to the default kindle interface, KoReader needs to be launched manually. I can’t imagine amazon not having a possible way to “blacklist” the KUAL applet or something.