The whole pin assembly acts like a miniature toggle bolt. The inner part of the pin assembly is actually a short stubby cylinder. When the pin is inserted into the sleeve, it presses on the sleeve and expands it slightly in the processor shield, locking it to the top divider shield.
I took a tiny drill bit and spun it into the center of the pin just enough so that I could insert a small sharp screw (I used a cup hook) into the pin. All it needed was just a slight tug, and the pin came out of the outer locking sleeve. I used the same screw to remove the outer sleeve.