Agreed. WD40 is for general cleaning and removing water from electrical componants (WD = Water displacing). You want a spray penetrant like plusgas or duck-oil. You also get stuff called freeze and release spray. This has chilling agents in it that make the metal contract as they evaporate along with a penetrating oil to get into the gap resulting from the freezing process.

Failing that, a well equipped machine shop will have a slide hammer with an expanding chuck on the end (designed for pulling bearings out of engine casings) that will grip the inside of the seat post and allow you to hammer it out.