The only neutraliser I'm aware of for salt is water. Unfortunately once the water evaporates you are left with the salt again.
If the carpets are dry then vacuuming with a really good upright vacuum cleaner, one with clean filters and new bag, should remove a lot of the salt crystals.
If you are wet extracting then the salt will wick back to the surface as the carpets dry. You could dry going with a bonnet system to dry off the top surface to prevent wick back.