[quote=@Onarax]
Actually Salt has been used to ward off evil spirits in a bunch of Asian cultures. Buddhism and Shintoism coming to mind in particular. 

A [url=https://books.google.com/books?id=vHjy-dPPUMUC&pg=PA65&lpg=PA65&dq=Buddhist+Salt+evil+spirits&source=bl&ots=7Jnw1ZYpoZ&sig=sweGK4eiltQxIGFCZlf0j9VGRZE&hl=en&sa=X&ved=0CE0Q6AEwCGoVChMIm9fiz4fDxwIVyZiACh38mw4E#v=onepage&q=Buddhist%20Salt%20evil%20spirits&f=false]random book from 1988[/url] makes it easily predate Supernatural (Which Spiderwick did already). I'm also pretty sure that there is some evidence in Roman Tradition as it would be used as an offering to the gods often times to keep malice out of the house. 

Christianity has also used it as a curse to keep all things out. 

"Abimelech fought against the city all that day, and he captured the city and killed the people who were in it; then he razed the city and sowed it with salt." (Judges 9:45)
[/quote]

Warding off bad luck and actually fighting demons with it are too different things. It was considered a symbol of purity as I've already said so using it in sacrifices or to stave off curses wouldn't be all that unusual. It was never a direct weapon against satanic forces until Supernatural though. 

That bit at the end about sowing the city with salt had nothing to do with religious practices or warding off evil. If you salt the earth it ensures that nothing will grow there again. In ancient wars it wasn't an unusual practice to salt enemy farms and cities after conquering if you didn't intend to take them as your own. It ensured that your enemy wouldn't be able to rebuild. At least not easily.