Ignition. So it won't start. Guess with no battery it wont start either.
I found these guys but they don't sell with out installation. http://www.ravelco.com/
Just wire it into the power wire of the starter and mount the switch someplace inconspicuous. Not going anywhere until you put the key in and turn it on. Rated at 300A cont/1500A momentary, so it would have no problem with a starter circuit.
Be careful when you disable an engine, it makes for fun diagnose when it quits on you in the middle of nowhere. And could make for a dangerous situation if it cuts out when driving.
Starter disable usually has the smallest potential for added grief with the biggest anti-theft benefit. Once the truck is started and running there is no danger of a starter disable causing engine stalling, which could definitely cause a poopy pants moment.
You can have a stereo/alarm shop add remote door locking and starter disable via an alarm for a modest price.
Also key fob starter disable setups are easily installed and very unobtrusive to use. Just plug the key in and the sensor by the ignition tumbler detects the special key fob and start the car as you normally would. No pre-flight checklist for valet or service techs or whatever.
I'll could tell you what I did, but then I'd have to kill you. No but in all seriousness , I used my cigarette lighter as my switch, and spliced it into the neutral safety switch. Push it in to start, pull it out to fool the fools. Works good!
In the early days many cars had a floor-mounted starter switch you activated with your left foot. My '29 Ford is an example.
A floor switch, located high and to the left (like the old dimmer switches on US cars), and possibly under a floor mat, could deter or at least delay a would-be thief.