Shooting technical advice
EEEEK! Dont even think about it! Whacking it like that can easily generate several tons of pressure in an ordinary engineers vice, and even a light grip is likely to force one or both barrels out of round, not to mention scuff the blacking as well.
Hitting a choke key with a hammer is likely to cause further damage.
So, rather than take any extreme measures, begin by giving the muzzles a really good soak in either penetrating oil or diesel; and I mean a good soak several days.
You can buy penetrating oil from car accessory shops like Halfords, Plus-Gas being a familiar brand.
Then try again with the key. If the choke tube still wont come out, take the gun to a gunsmith and get him to remove it.
I have heard of choke tubes being stuck in so tightly that they have had to be split internally with a chisel a job which requires a lot of experience.
Once the tube is out, you can stop this happening again by taking the tubes out every time you clean the gun, carefully cleaning the threads, and re-inserting them with a smear of grease.