Why did Dumbledore never try to recruit a ghost for the Defense Against the Dark Arts subject? Wouldn't that have overcome Voldemort's curse? I mean, what can you do to a person who's already dead?
You can petrify them with a Basilisk, for one thing:
They could become too depressed to be much use to anyone, for another:
They could be trapped inside a haunted mirror:
They could be expelled by the Ministry’s Spirit Divison of the Department of Regulation and Control of Magical Creatures (in Myrtle’s case, they made her go back to Hogwarts—presumably they could do the reverse) :
GoF ch 25: "Oh yes, very easy to forget Myrtle's dead," said Myrtle, gulping, looking at him out of swollen eyes. "Nobody missed me even when I was alive. Took them hours and hours to find my body - I know, I was sitting there waiting for them. Olive Hornby came into the bathroom - Are you in here again, sulking, Myrtle?' she said, 'because Professor Dippet asked me to look for you -' And then she saw my body...ooooh, she didn't forget it until her dying day, I made sure of that...followed her around and reminded her, I did. I remember at her brother's wedding -" But Harry wasn't listening; he was thinking about the merpeople's song again. "We've taken what you II sorely miss." That sounded as though they were going to steal something of his, something he had to get back. What were they going to take? "-and then, of course, she went to the Ministry of Magic to stop me stalking her, so I had to come back here and live in my toilet."