first they never kill a ghost just with salt, they are need spread salt on the rests(bones, body or something too appreciate by the ghost) of the ghost, then burn the rests.

on repeated occasions, some one ask to the Winchester why use salt or iron bar's to repeal ghosts  ? they answered, because are pure materials. Then I think diluted salt does not work like pure salt

in latest episodes you can see, in order to a ghost can move a object they need a big concentration or /and a huge rage, when something pure "touch" the ghost, this lost the ability to move objects (or even possess) a small period of time.

in the end of the season seven you can see all this things

Firstly, they never kill a ghost with just salt, they are need to spread salt on the remains (bones, body or something associated with the ghost) of the ghost, then burn the remains.

On repeated occasions, someone asks the Winchesters 'Why use salt or iron bars to repel ghosts?' They answered, 'Because they're pure materials'. So I think diluted salt would not work like pure salt.

In later episodes you can see, in order for a ghost to move an object they need a to concentrate and/or be enraged; when something pure "touches" the ghost (iron, salt etc.), they lose the ability to move objects (or even appear corporeal) for a small period of time.

By the end of the season seven you can see all these things.