It is important to recognize that even though we, as adults, know that there are no such things as ghosts, to the child they are real. And no amount of logic will convince them otherwise. You have to accept, for a while, that what they think is real, is actually real, and then you can deal with making it not scary.
So rather than trying to reverse their thinking, try to control it and direct it. My son (5 years old at the time) was sure there were ghosts. So we talked about "How do they get in the house?" Apparently they can come in the windows. What he didn't know, and I did, was that it you put a piece of tape in the corner of the window it blocks the ghosts.
He was also afraid of vampires (did I mention I hate Halloween). He was afraid they would get in his room. What he didn't know was that vampire are afraid of blue carpet... oh did I mention his room has blue carpet in it.
I couldn't convince him that neither one was real, but I could divert and redirect his concerns until he was more rational and we could talk about make-believe and reality.
I hear that if you put a piece of tape over the corner of a mirror it blocks out ghosts just as good as it does on windows.
Hope this idea works.