It's their machine. Who are YOU to decide that their task/job today is less important than a theoretical security threat?

I think that at the "Update 1" (warning, "press a key to continue at your own risk") level the message should mention a future date certain (the "Update 2" date), upon which the system won't boot unless it has been updated. That makes punishing them by rendering their computer unbootable, on that date, less unreasonable than doing it with no warning.

Dave