As K bikes age its not uncommon for the threads on the seat locking post to become corroded and seize. Since the threaded part it screws into is mounted in rubber you can't just reef on it to break it free as the likely result is that you'll just tear the threaded part loose from it's rubber mounting.
Here's my solution:
Take the seat post and it's mount off of the seat.
From behind use the handiest tool in the world (a Dremel cutting wheel) to cut into and flatten two opposing sides of the threaded metal insert so that you can get a good purchase on it with some Vise-Grips. You can then exert more torque on the bolt to break the seized threads free without risking tearing the threaded part free from the rubber. (Soaking the threads with some penetrating oil ahead of time doesn't hurt.)
See attached pics....
