use a shimano (or other brand that isn't campy) front brake that has quick release lever. Set it up so it works when the lever is tight but not when it is released. Flick it off when you lock the bike and do it up before you ride off. If the thief rides off without noticing then the brake won't work and they'll need to be good at leg braking.
use mountain bike or BMX brake levers that allow you to unhook the brake cable at the lever end. Unhook when you lock up and do up up before you ride. Similar results to the idea above.
Two quick ideas:
use a shimano (or other brand that isn't campy) front brake that has quick release lever. Set it up so it works when the lever is tight but not when it is released. Flick it off when you lock the bike and do it up before you ride off. If the thief rides off without noticing then the brake won't work and they'll need to be good at leg braking.
use mountain bike or BMX brake levers that allow you to unhook the brake cable at the lever end. Unhook when you lock up and do up up before you ride. Similar results to the idea above.