Park has a good description of different ways of taking care of this

https://www.parktool.com/blog/repair...osts-and-stems

If the PB blaster and/or trying to cut it out doesn't work, then it gets really tough.

At that point, lye can be used but that can also be dangerous stuff to work with.

Here's a thread,

Stuck Seat Post? Lye your way out of it!