This worked for me on two different seat posts. First, let penetrating oil soak in for a few days. If the frame is stripped, or at least the bars, wheels and pedals or cranks off, so you can lay the bike flat on the garage floor or drive, put the biggest crescent wrench you can find on the flats on the top of the seatpost.

STOMP HARD!! It may take a couple of stomps, but in both cases a good hard stomp did the job.