Before you folow any of the more invasive and time-consuming suggestions, take it to your local auto repair shop and have a mechanic use a heavy-duty pneumatic hammer on the underside of the saddle clamp of the seatpost. Ten seconds and done.

Note: don't clamp the frame in a fixture first---just hold the frame up by hand while the pneumatic hammer is slamming the saddle clamp. Clamping the frame solidly will absorb most of the impact, rendering the air hammer less effective, and might result in damage to the frame.