If you really want to get rid of it, seek out a company that does fire restoration work. The one's around me have what they call an "ozone room", where ozone gas is pumped in. The ozone gas breaks down the carbon based smoke residue, and kills it. That will probably require a week of the instrument sitting in there, but it should be pretty inexpensive if you haggle with them.
I had a house-fire a few years back, and they got all the smoke odor out of my acoustic instruments nicely.
"If you hit a wrong note, then make it right by what you play afterwards." - Joe Pass