Jet-A (commercial avegas) is already about as nonvolatile as a petrochemical can be and still be a useful fuel. There is no way to spontaneously change the chemical formulation to something that won’t burn. You can, of course, add fire suppressants which essentially starve the fuel by displacing oxygen (CO[sub]2[/sub] or Inergen), bind with the free hydrogen ends to prevent oxidation (halomethanes), or just absorb energy to reduce the reaction below a sustainable threshold (pentafluoroethane). However, this requires about as much suppressant as it does available fuel and oxidizer, which would obviously be impractical to carry on an airplane.
Realistically, modern commercial aircraft are about as safe as materials, controls, and avionics technology are able to make them at the current state of the art. Despite the fact that an aircraft performs an essentially impossible function–keeping tons of loosely aggregated material suspended in mid-air for hours at a time–the rate of actual failures is so low that even if you travelled by aircraft on a daily basis you are more likely to suffer a serious injury at home or driving to work. To the untrained eye, it may seem that this is because building, flying, and maintaining aircraft is a simple and routine task, but a more-than-cursory look at the entire chain of design, testing, training, inspection, maintenance, and regulatory control shows that this record of safety comes not from the innate lack of hazard but an unreasonably complex and highly engineered overarching system. The general public gets upset when a plane crash occurs because pilots weren’t trained for certain hazard conditions or an incipient failure in an elevator control actuator wasn’t discovered, but really, given the complexity of a single aircraft, much less the systems that let dozens of them land and take off at an airport in the span of an hour, it is phenomenal that they aren’t falling out of the sky every day.