There's a lot of romantic hogwash about cleaning cast iron. Just grab a regular scrub sponge and some dishwashing detergent and hot water and scrub until it's clean.
If this does any damage at all to the seasoning, then the pan wasn't really seasoned. To get seasoning off, you need chemicals the equivalent of dishwasher detergent or oven cleaner, or abrasives the equivalent of sandpaper. I've removed seasoning from manky old pans ... it took two days soaking in a cocktail of dishwasher detergent and draino.
That "seasoning" is a matrix of polymerized oil and carbonized oil. Polymer=plastic. Tough and nasty stuff. Which is why you want to apply it in nice thin even coats in the first place, because you'll suffer trying to get any off.
Incidentally, this is one reason to be careful using teflon pans on any kind of high heat, especially if you're using an oil that's high in unsaturated fats (canola, safflower, sunflower, etc.). If any of that oil polymerizes on the teflon, you may never get it off. I don't know if there's anything that will attack the polymerized oil that won't attack the teflon coating harder. If anyone knows, please share.
One problem I've discovered with using non-stick pans solely for what they're do best is that I end up using mine about twice a year ... the rest of the time it's on a wire shelf, where, thanks to my terrible range hood, it gets a mist of airborn oil every time I saute something. When I looked at it the other day, the oil had oxidized to the point where it wouldn't come off with hand washing detergent. It eventually came off off after soaking for an hour in a solution of dishwasher detergent. I don't know if this is harmful to the coating. Luckily, it's a $15 pan (see above!)
