This is the right answer: Break the fork link. I sometimes do this to make a private "fork" of a public repo so that I can add my own notes about how to use it, remind myself what happened when I tried it, add a config script for my own peculiar setup, etc.

It's unfortunate because not having a "real" fork makes it harder to send pull requests and track the upstream. But it's sometimes necessary to get around stupid github policies.