Soak it in your fluid of choice. I've never heard of that ammonia suggestion, give it a go and tell us how it works.

Don't go hitting the thing, you're not after a sudden jar. You want torque and you get that from a long arm. Seeing rotafixing hasn't worked, I'd clamp the cog in a vice, then use the rim to turn off the cog (and make sure you ARE turning it the right way, it's easy to get it wrong when doing this). You'll probably ruin the cog but if you go swinging hammers around or trying to put long levers on chain whips, you're more likely to break something in the wheel.

Best bet, take it to a bike shop. They'll probably do it while you wait and won't charge much unless you've a real problem in which case, you probably won't get it off yourself anyway.