While secured cockpit doors are not required on chartered or corporate aircraft, they do have a few security measures which are the same as those found on airliners to deter criminal activity or hijacking by passengers aboard the aircraft:
Security screenings: Large aviation charter companies will subject travelers to TSA screening’s, involving metal detectors and x-ray luggage that they carry board.
Higher class clientele: Travelers a board private jets and charted aircraft generally are wealthy individuals, or well-connected ones usually are not up for causing that kind of major crime aboard an aircraft.
Traveler reputation amongst charter companies: as mentioned earlier large scale criminal activity by most private jet flyers is rare, but there can be cases of obnoxious passengers being rude or abusive with the flight crews and causing minor property damage. This is particularly troublesome with celebrities, musicians, professional athletes, etc. That kind of antisocial behavior will be quickly curbed as such people develop a reputation amongst flight crews and charter companies, as they have quite long and vivid memories. The bottom line is if you don’t want to travel by car, bus, or amongst other “little people” on an airliner, be kind to your flight crews and they’ll be kind to you.
The fear of the Law: Causing trouble aboard an aircraft or anything that could be interpreted as a violation of FAR 91.13 can carry sentences of up to 10 years in prison, and fines as great as $250,000. Aircraft hijacking is a capital offense under the US code.
Flight crews pack heat: Yes some of them are armed with guns and will shoot you if they believe you are an immediate threat to the aircraft or other passengers.