There is no way to permanetly disable the vehicle theft security system. If he told you there was, he was incorrect.
When you lock the car, it arms. The only two ways to disarm it are by unlocking with the doors with the keyfob, or starting the vehicle with the key. The immobilizer portion of the key fob will work, even without a battery.
The horn will blow when you open the door, but after the truck runs for about 5 or 10 seconds, the horn and alarm should then shut off.
This applies to vehicles equipped with factory security.
I hope this makes sense?Leo