Cops can turn off your car remotely, but only if your car has a remote turn off and remote disable device installed on the car that is connected to the ignition system.
The police themselves cannot turn your car off remotely if you have the remote disable device on your car, but instead the police can contact the company that operates the remote disabling device like Onstar and have them disable the vehicle and turn the vehicle off remotely.
Police have had vehicles remotely disabled, when they've been reported stolen and even when the vehicle has been in a pursuit.
But without the vehicle having the remote disable option and device installed on the car, the car cannot be remotely disabled.
Having a remote disable feature on your vehicle is a good option, if you get the vehicle stolen as it can prevent it from being driven and it can allow the vehicle to be shut off so that the police can catch the person who stole the vehicle.