Start by taking the clamp off so you have easy access, then proceed to spray PB blaster into the non-existent space between where the post slides into the frame. Let it sit there and try to do it's work and repeat a few times throughout the day. Try a little bit of twisting at the end of the day and see if it worked its' magic. If not, grab a pipe wrench or channel locks and grab the post just above the collar and put some force into a slight twist back and forth. If none of this works, there are more drastic options like adding heat or cutting the seatpost off and then cutting a slit into it from the inside, but try to avoid those.