If it won't move and a Gunsmith can't get it out, the only good option you have is to have a machinist put it in a lathe and bore it out. When it gets thin enough it will collapse, and you will be able to see if the threads in the bore are damaged.
What kind of shotgun is it and how long has that choke been in there?
Unfortunately, depending on what it is the cheapest fix might be a new barrel.