The pen is pre-activated by being taken out of it's owner's area (RFID, positional sensors, whatever).
An eccentrically shaped barrel ensures that every user grasps the pen in exactly the same places. Moulded conductive plates make up the portions of the barrel that contact key musculature in the
user's hand.
The pen is then activated by the nib touching paper: the ink flows, bridging a switch.
A microchip controlled battery activates the muscles of the thief, causing him/her to write "I shall not steal office supplies from Flying Toaster" until the pen is lifted from the paper, the ink runs out, or the pen is returned to the original owner's demesne.