Holy water. Loads and loads of holy water.
Also: from ghostwalk (note: technically the ghosts ghostwalk refers to are entirely different, non-undead creatures. Your DM is being a dick, though, so I'm going to ignore that.)
Soul Venom: Harmless to living creatures, but acts as Large monstrous scorpion venom (injury DC 18, 1d6 Str/1d6 Str) against manifested ghosts. 200 gp
Cretchwater Oil: Against living creatures, acts as Small monstrous centipede poison (injury DC 11, 1d2 Dex/1d2 Dex), but against ghosts, it functions as greenblood oil (injury DC 13, 1 Con, 1d2 Con). 150 gp
Dark Ectoplasm: Poison, affects only ghosts (injury DC 18, 1d6 Cha/1d6 Cha). 700 gp