I ran into this once. The following only will work if the BB is open to the seat tube. I assume it’s already been sprayed with a decent lube.
I ran a piece of all thread down through the post, put a washer on the bottom big enough to catch the post, then a nut. Then an oversized washer on top of the seat tube, and a nut on top. Grab the all thread with vise-grips, and turn the top nut to pull the post up. Eventually it will hit the top washer, but by then you can probably just yank on the all-thread to get it out.