Terrible idea. Just terrible. And, thankfully, impossible. Even if it was possible some hackers would find a way around it in 24 hours or less.
I'm trying to imagine it...all cars come with a built in signal scrambler. Disconnect the scrambler and pay a fine. And a whole new industry crops up for the enforcement of maintenance of said scrambler. Think about the smog check industry and multiply it by a bazillion. Just what we need. It's like TSA for auto travelers.
Not to mention passengers, being trapped in the car, etc. The things cell phones are good for would be made moot.
I know, let's just come up with a chip we can implant in people to stop them from doing anything stupid. Who's first??