This add-on allows you to lock and unlock vehicles! Type /lockVehicle and /unlockVehicle while in your vehicle to lock and unlock it, respectively. You can also use the Key Car item to do this as well (requires Item_Key, which is default). If a vehicle is locked, only you or an admin may enter your vehicle.
Type /lockVehicles and /unlockVehicles to lock and unlock all of your vehicles, respectively. You can also put a name after the command if you are an admin to lock a specific player's vehicles.
Admins can type /lockAllVehicles and /unlockAllVehicles to lock and unlock all vehicles on the server, respectively.