I was an airline mechanic for 19 years and have been taxi qualified in many different models of Fokker, Boeing, and Airbus. These aircraft do not have door locks, wheel locks, or ignition keys/locks. The security is a big fence, cameras, roving patrols, airport workers, etc. This guys job was moving aircraft around for parking, etc. Even if they had locks, he'd have the key.
I've thought several times that if I was willing to kill myself, there would be nothing preventing me from taking one of these airliners and taking off. Many times I've done high speed taxis on the runway (at night) to troubleshoot pilot write-ups for things like nose wheel shimmy.. There would always be two mechanics driving the plane but that was just for company rules. Just about on any given night I could have taken a plane. There's no way to prevent this. Prevention starts by the job interview and 10 year background check. Then years of service as well as classroom training. At that point if a guy goes nuts you can't place blame on anyone except him.