Ooh, I like that pain beam. Perhaps one in the seat 8)? I'd jump off FAST like if my (how do I put this lightly?) NUTSACK started burning.
In terms of what you could REALLY do, something along the following:
Redundant hidden switches. One under the seat, one right next to the derailleur, etc.
Plug braking. If all the switches aren't turned on, the motor remains attached to a high-wattage resistor. Make it hard for them to leave.
Removable batteries. Not all of us are able to do this, but if you can take your batteries with you, do it. Especially if the cost more than the bike.
Take the seat off. I had to do this once because of a faulty quick release, and I can tell you: Riding without a seat is novel, but not fun.
I also have an interesting idea for an alarm that could be used with the plug brake. Across the terminals of the plug brake, attach a capacitor, small bleeder resistor, and a zener. Hack it into the activation circuitry of your favorite alarm. Now, when the bike is just touched or moved in passing, it does nothing. BUT if the motor is rotated a bit, a voltage differential is produced, charging the cap to the point of being able to reverse bias the zener, activating the alarm. I would imagine that someone who actually gets the bike off a rack and THEN has to deal with a bike that's damn near impossible to move and has a screaming alarm going off will think twice about going any further. Considering how much some of our batteries weigh, I don't think they will be able to carry it very far, either.