Install an alarm with an ignition cut out.  Add power door locks while you
are at it and one click of the fob and you are locked, alarmed and disabled.
I just installed this system myself, parts including electric windows ran
about $340, it's fantastic!  Hi quality Swiss made lock actuators, Italian
made window winders, and a Chinese alarm system (Avital). We travel in major
cities so it's important to me to have an alarm to deter smash and grabbers.
A van full of stuff is just too tempting. Best mods I've ever made.

Stuart