I've been around some negative beings (as well as good ones) for a long time, and for awhile now I've had salt lines around my room for protection, and I was a little curious as to why it deters them. Did it create an impassible barrier? Did it simply purify their negative energy so they would avoid it? While out of body, I asked them what it does to them.
The shape shifter (who took the appearance of my brother) let me take a pinch of salt and toss it at the side of it's face, and I felt what it had felt.
It is like getting hit with a mace with a bunch of sharp spikes on it, and the lasting effect feels like pins and needles/tingling with intense soreness. And this was just a pinch of salt.

It feels pretty horrible for them, and not in a way that would make them angry, so I would definitely recommend salt as a way to deter evil spirits.