Air chisel. With a "Punch" type bit in it.
Already been suggested, but I want to second it.
I have one ground down for exactly this job.
Put the nut back on a few threads to help keep the bit centred on the end of the bolt and to prevent peening the threads and/or restore the threads if it mushrooms the end of the bolt slightly.
The reason this works when the BFH doesn't -
The bolt is frozen in the steel sleeve inside the rubber bushing.
When you hit it with a hammer, the bushing absorbs the impact and you aren't getting any real force on it. The vibration of the air chisel pushes it out bit by bit, because it is hitting at so many blows per second, it moves the bolt faster than the bushing can rebound.
Make sense?
I have watched people beat the beejeezuz out of a spring bolt and get nowhere. Then come over with my air chisel and push it right out.
This info - 2¢
Getting the bolt out when the BFH won't - $39.95 (The price of a cheap AC)
The look on peoples faces when it comes right out after they have beat on it for 20 minutes ---- Priceless.