Get a bigger chain whip
.
Seriously, and it pains me to suggest this, get a [size=50]Shimano DA[/size] chain whip and increase the leverage by sticking a monster tube spanner on the end.
There is also a method (can't find the link at the moment) which involves removing the wheel, taking the chain off the crank and double-wrapping it round the sprocket. Put the wheel back on and then grab the wheel rim and try and turn it. The chain will hold the sprocket and you should get enough leverage from the wheel to unscrew it. You might just end up taking a load of paint off your chainstay though.