I did this on my RV-7 a while back.
I applied Mouse Milk in the bolt hole, and tapped on the steel a bit. I then used a butane torch and pointed the flame "close" to where the tail spring went into the fuselage. I used the aft most bolt holes as leverage points to twist and/push-pull the spring.
It took a bit, I found that applying a "shock" twisting moment broke it free.