Oh, that was not even my point. You are talking about the possibility of passing data after having paid. I think there is no way to prevent people from doing so -- the industry has decades of years experience with pirated licensed software. So you better don't waste your time for implementing "preventing forks" on your platform --- if I pay on your platform for getting access to a repo which I can clone on my computer, I will push my copy to some other remote, of course.