If you can listen in, you know the frequency. What stops people from hopping on the channel and chatting away?
I would wager that there would be about 150,000 hot, sweaty, angry NASCAR fans who would recify that situation right quick. NASCAR is a very self-policing sport.
It depends on what band they’re broadcasting on. Some bands involve special, relatively sophisticated equipment to transmit. Not all equipment is portable. Plus, interfering signals can be located pretty easily and the offender can be, ah, persuaded to knock it off within a few minutes.
In any event, interfering with a broadcast is against FCC regs and (possibly, but I can’t verify it until tomorrow) criminal law. Given the fact that NASCAR uses their in-car radio systems to broadcast safety information, I’d be willing to bet that the FCC would take an interference complaint seriously. Penalties are pretty stiff for this, and if interference were to cause crash, there’d be massive hell to pay.
Robin
Oh, and the wiseass answer is that the average NASCAR isn’t bright enough to figure out how. 
::MsRobyn runs somewhere safe. Very fast::
Robin