If all else fails, buy/rent/borrow/steal a blowtorch. Heat up the cog, especially around the threads. That sucka WILL come off.
Try the Italian technique to take the cog off, you will be surprised at how well it works. I used that twechnique to tighten down a cog on a bike, and I was able to get the cog at least 1/2 turn tighter than I could by riding the bike or by a chain whip...
Make sure that you are turning the right direction, though. And use a rag or something to protect your bb shell.