Maybe try baking soda carpet cleaner, liberally applied, then cover the thing with a blanket for a week. don't sit on it.
then get a good vacumn and suck it all out that will come out. Might take a couple of tries, but it'd have to be better than something liquid, that might set the smell in.

On the other hand, a search of the net found me this product. I never used it, but it sounds, if the ad is believable, like just the product that'll work.

www.scoe10x.com/Scripts/default.asp

But if all else fails, and the couch is in otherwise fairly good condition, find someone who'll buy it for a garage, or some folks that smoke already and don't mind the stench.

But if you get rid of it, be sure to get all the change out of it.
;)