Kroil. Way better than wd-40. If I were in your situation, I would soak some (whatever viscous lubricant you have handy) into the threads of the cog/hub interface, then take a rubber mallet/hammer/beating stick and give it a few good taps to try and break the seized parts loose. You'd be surprised what a few good whacks can do for stuck parts. Also, if you're planning on scrapping the cog, take a cold-chisel and a good solid hammer, place the cold chisel on one of the teeth, hitting it so that it is loosening the cog, and try to break it free that way. A few quick blows works better on seized parts I've found. Good luck. Oh, and how cheap are we talking here (for the set of wheels, if you don't get it off)?