Wildwood 

Welcome. Check threads in C&V.
Usually, for the uninitiated, the first rule is: do no harm. Not useful to get it unstuck by buggering the seat tube.

Best result is to remove chainset, open the bottom bracket, flip the bike and come at it from the inside with something much stronger than WD40's std product. Soaking overnight may help a stubborn problem with a struggle-less chemical solution.
I have heard of using a piece of cloth tied at the post/tube junction and soaked with your chosen chemical liquid, .......sometimes for days.
Beyond that = there are various mechanical methods. Not always preferred, but moving in the direction of 'no other solution'.
Can you do that? Tools?

I now see 5 posts listed, so you probably have your answer.