Removing that bloody sprocket

vice
clamp the cog and turn the wheel right out near the edges / holding rim ... huge leverage / much more torque
heat cog with blow torch different metals expand at different rates
just pray the the hub doesn't expand faster than the cog otherwise it'll just tighten even more !! DOH !

Coefficient of thermal expansion for aluminium is higher than steel, so if you heat up the cog, the heat transfers to the hub and it expands more than the cog so tightening it. The only benefit heating has is you need to break down the loctite used on the cog if you have a Fuji Track!

As previously suggested, wrap a chain round the cog, hold in a large vice and turn the rim. It will come off. If you're happy to sacrifice the cog, you can beat it off with a hammer and chisel. Both methods work, I've tested them!