Several of the RC electronics manufacturers make them and they're called "Failsafes" and will do as you inquired, but are primarily used in nitro vehicles though they can be used in electrics if the need arises.

But my question to you would be "what's leading up to and/or contributing to your runaway problem?" Do you have a jumbled mess of wiring, power leads crossing the antenna wire, low battery packs (in the car or the transmitter), or are you running in an area predominated by overhead or underground power and telephone lines? Because all could be the potential culprit, and even having a 2.4ghz radio isn't the cure, but neither will a failsafe ~ and it's often easier cured tracing down the little things you're doing wrong or making a mess of than looking for for a miracle cure to what ails you.