As mentioned in the thread, short of not being connected to WiFi while being connected to a power source, there are no controls to stop an automatic update download at some point. You can do something like install a tvOS profile which will prevent that (and make sure to delete the already downloaded update).Last night my IPP 10.5 using 10.3.2 asked me to install iOS 11 right after downloading 1.6 GB without my consent. I almost hit OK, the popup appeared right after I opened the Smart Cover. It seems it doesn't matter saying NO and in iTunes choosing the option to not be asked again, over time the device will download the update and bug us again.
Hope this doesn't happen anymore, maybe I visited some internal option without noticing.