i would think the easiest option would be to run the live feed from the stop solenoid into cabin area and onto a two pole micro switch,hide that somewhere out of way (under dash or even in glove box)then return the wire off the back of the switch back to the solenoid.when you turn engine off flick the switch and even if they "hot wire"or "Black box"it it wont start because the switch is stopping the feed to the solenoid.its a very simple mod thats very effective given that most car theives are brain dead morons and not likely to lift the bonnet to diagnose a non starting fault,obviously it wont stop them towing it though.
hope this helps,
Gareth