Blocking/blacklisting the service Amazon Device Client Platform (ADCP)
from using Wi-Fi with something like Android Firewall (Requires Root)
should be enough to stop any Amazon OS Updates.
I'm not sure of it's APK name in /system but renaming it to something like "name.apk.bak" may just be as good but might cause system stability issues if you still want to use other Amazon services on the device, for that I'd suggest using the above Wi-Fi blocking method in any case.
It certainly works on the 2012 model, I'm still on 7.2.3