A low tech solution is to use a laptop that doesn't have a battery and keep it plugged in all the time. When they grab the laptop the cord invariably comes unplugged and the thing turns off. Bonus points if you have a fake battery so the weight doesn't give the game away.

A process that logs out active users, kills all their processes, and wipes decrypted storage keys from memory when the computer is separated from its owner (probably detected via bluetooth phone/wristband)?