There was another thread about stubborn cogs, maybe 3-4 years ago. Depending on what you are willing to sacrifice vs. trying to save, there are some tactical to nuclear options. In one case, I used a cutoff wheel on a dremel tool to cut through the stuck cog. My preferred method these days is heat, e.g. from a propane torch, used judiciously so as not to compromise the heat treat of the cog being heated. Don't worry about coking the oil/grease inside the freewheel; that can be cleaned up, assuming you are willing to take the freewheel to bits afterward.