Indeed, the 'kill signals' or more correctly EMMs (Entitlement Management Messages)
are sent via satellite, so to miss them the box would have to be powered off or disconnected from satellite feeds.
The messages are also sent at random intervals over several weeks/months, so trying to avoid them would mean not using the box for this period, and even then, the EMMs, which are stored on the card, automatically expire anyway.
So in short its a lot of hassle, on the off chance you might get PVR functionality for a few months after cancelling.
Then of course, there is the legality of deliberately trying to circumvent Sky/NDS CA system....