Well one way would be just like how Android phone manufacturers are doing it now... with locked bootloaders and binary blobs. Even current GNU/Linux phones still largely need blobs to work properly.
This is misleading. The blobs are only in the firmware, not in the OS, not in the bootloader, not running on the CPU.
Having a technical possibility to lock down GNU/Linux phones in principle in undefined future by undefined entity that doesn't even produce them yet is a FUD argument.
