Removing stuck or seized seatpost?

the only way is to wind it out while pulling. whacking it with a hammer won't work.

The trick is to ensure you're applying force in both directions at once - if you can only twist the post, you need to either apply a constant pulling force while twisting, or at least find a way to deliver some taps or whacks while twisting.

So whacking it should work, but only if you're twisting at the time