In Dresden, ghosts have the capacity to just go away back into their Demesne, whether they are Taken Out or not, so the salt could be (as many others have suggested) a Compel to simply go away, if the net result is only giving the heroes an exchange or two of breathing room. Anything longer than that, however, and I feel the Catch would need to have been used as a component in an attack that forced a Concession, or resulted in a Take Out.
My breakdown would be:
Ghost disappears for 1 exchange: Compel the Ghost's vulnerability
Ghost disappears for 2 exchanges: Compel + Fate Point
Ghost disappears for long enough that combat is effectively over: Ghost has Conceded the Conflict to avoid being Taken Out
Ghost gone forever (or as long as it will ever be gone): Ghost has been Taken Out