Sorry for replying to a very old thread, but I found this via search when trying to solve this problem of a "stubborn" drive sprocket. As Richard threatened, I actually ended up taking the grinder to mine. Using an angle grinder and a Dremel with a cut-off wheel, I found my problem when the first of what ended up being three pieces of the tapered sprocket hub finally came free.
It was readily apparent that the taper had slipped and the sprocket friction welded to the tapered shaft. Sometimes a bigger hammer and more heat isn't the solution. At least I didn't destroy anything else in the attempt. Maybe this will save some else some grief.
DJinCA