How to remove completely stuck saddle post from frame
Penetrating oil (Liquid Wrench).
It may take a few days as it needs to work its way down.
Or it may free it up in a few minutes.     

The risk you take with a lot of force with a pipe wrench is you bend the bike or brake off the post or get the post out of round.  

You should grease the seatpost to help prevent this.  If you don't bend the post then I would reuse it. 

It is most likely rust.  You cannot hammer in an over sized seatpost.  

If you do sacrifice the seatpost and use a use a lot of force with a pipe wrench then don't have the pipe wrench close to the bike as you could get the post out of round and really jam it in there.  That may have happened when you got it to move a bit and then jammed.

You have a couple comments of use heat and I don't disagree but I would hit it with penetrating oil for a couple days before I resorted to heat.  Not just once.  You need to hit every few hours with oil.  If you do go heat then heat gun before torch.