'Free Private Preston!'
Was what we used to see as graffiti in the Washington, D.C. area in the early '70's. He was some jerk Specialist who stole a US Army Huey and landed on the grounds of the White House to prove that he could, too, fly a helicopter after the Army had told him he was not even helicopter pilot material! After that the Army fitted, yes, ignition keys!
Ignition keys for airliners, Bubba? Whatever next?
There's no ultimate protection against this sort of thing. I just did a TSA on-line security course while renewing my FAA CFI. There were some pretty obvious holes in the cheese slices there to be seen, I have to say, but then there would be! At some point you are still going to see someone in sole charge of an aircraft, when there must be an obvious risk involved. Well, every day we share the road with numerous vehicles that go speeding past just inches away, not seeming to worry about the odd homicidal lunatic bent on causing a head-on crash. What about that, then?
At some point one simply has to trust people to behave properly. We have seen thefts of cars, of course, but also aircraft, buses, boats, the odd train or two, an M-60 tank, diggers, bulldozers... pretty much anything you can think of can be misused to commit mayhem.
Somehow you figure out a way to put a 'foolproof' ignition switch into the cockpit, ignoring the obvious risks that must bring of somehow shutting down vital systems in flight... so your maniac simply breaks into the key cabinet, say. What then?