Deterrents against partial (component) theft Big question: Is there any way to increase the security of bolted on portions of a bicycle? 

Specifics: I cycle as my primary mode of transport (in East London) on a early 90s steel road bike with drop bars. It's a good bike, though it could use a paint job. A few weeks ago my bicycle was securely locked in a place with moderate foot traffic (I'd guess someone walked by every 3-5 minutes) for about 4 hours. During that time, someone undid my (quill) stem and the brake cables (at the caliper) and made off with my handlebars, stem, brake levers and brake cabling. So with that in mind, does anyone know of means to secure that portion of my bicycle?

(Part of my concern is that I'd love to move to STI/Ergos from my current downtube thumb shifters, but I can't escape the nagging feeling that a move like that would have massively increased the replacement cost of this theft, and I'm yet to find an acceptable security strategy)



Update:
Was just directed to https://pinheadlocks.com/store/en/  by jstutters on twitter which is clearly close to what I'm after, though not quite right for a quill stem. If anyone has experience with this sort of system, I'd love to hear about it and if anyone knows of something similar that works with quill stems/threaded headsets that would also be very useful·



Second Update(Nov 2014):
I've taken to using a full setup from http://atomic22.com they're pricy, but exactly what I was after. Been using them for a couple months now and am thus far very pleased.
For quill stems, you can use a rubber cement to glue a ball-bearing into the socket on the stem bolt, making it so you can't get at it with an allen key.

You'd obviously want to use a glue that can be removed with a solvent, so that you'd be able to do regular service.