You can say a spell or use the potion. The potion however needs to be poured over the remains of corpse.
Ingredients for potion:
-palm full of Pomegranate Seeds ground to a paste
-Pinch of comfry root
-Pinch of blood meal
-Pinch of Clematis seed
-Pinch of dried rubarb leaf
-Add mix of Twice Blessed water
-Bring all to a boil
Pour over the remains of ghost
Spell to Banish Ghost
