The seasoning can be ruined, but you really have to try hard. I had a roommate who would cook steaks on my cast iron skillet after marinating them in steak sauce. The sauce would burn on the bottom of the skillet and leave a layer of charcoal, and then he left it soaking in the sink full of soap, which made it rust. I ended up taking the pan to my friend's metal shop and sand-blasting it back to bare metal and starting over on the seasoning. But even then the iron was intact, you'd have to hit it with a sledgehammer to permanently destroy a cast iron skillet.