Yeah, the "pacemaker" thing is bunkum. cars (and pacemakers) have to go through pretty stringent "Electromagnetic Compatibility" tests before they can be sold. There are lots of things on cars (ABS, ESC, throttle by wire, etc) that could kill people if they were susceptible to electrical interference (within reason)! As Teabelly says, the technology exists. It won't be impregnable, nothing is, and you'll just get people "chipping" their cars to remove that feature. There will then, presumably, be the introduction of a check on its operation at each MOT - which means MOT stations will be equipped with the remote "kill" devices, and then, as you say, the scrotes will get their paws on them. (Presumably police cars will be exempt from having them fitted)?
Anyway, the technology is there. You can even do it via a mobile phone if you want - to your own car, but it's only as good as any other immobiliser.