Most cars have digital shifters, push-button ignition and "steering assist" these days. If some three-letter agency wanted to remotely control your car, then they can.

The best defense is to go to any car modification shop and ask them to install a fuel line shutoff switch on your dashboard. It's completely independent of the car's computer system and no remote control can affect that. If the vehicle begins accelerating or doing something unexpected... then immediately hit that switch and turn off the fuel. No fuel = no acceleration. Engine immediately shuts down, so the alternator doesn't provide power to anything. Power steering stops. You'll still have brakes and (even if remotely controlled) the car will eventually stop. Also, remember that the emergency brake is manual. It isn't connected to the computer. You can use an emergency brake like a normal brake... if you hold the lever or button while applying the brake.

The sad thing is that this is even a topic. We don't have any faith in our own government anymore, and every one of us knows they do this stuff.