I used to be afraid of a boogey man in my closet as a kid, and even sort of into my teens. When I figured out that 'the boogey man' was my mind assigning something tangible to an intangible fear, the fear of the boogey man went away. The 'boogey man' in my closet was actually the deep seated fear in my subconscious of my father, who was prone to enraged outbursts and making the family feel fearful.
When my 7 year old daughter surprised me one night about a ghost in her room, I didn't try to trick her with ghost spray and putting tape on the window. I didn't delve into the spirit world and get all mystical on her. I stopped what I was doing (working late at my computer), and told her that number one, fear is something that is part of being human, and to some degree, it's an important thing to have if you want to survive. I told her that if there wasn't something that made her fearful every day until the day she passes on, then I'd be really concerned.
Fear is part of the package when it comes to being human. It helps you look both ways before you cross the street. We evolved and survived because fear kept us alive. Then, I told her that 'a ghost' is just her mind trying to put a picture on something that's bothering her that she can't quite explain. So what was bothering her? I let her talk. Turns out her older brother was changing and didn't want to play with her anymore. Turns out he's taken to yelling at her a lot when I'm not looking. Turns out that she's having trouble making friends, and she's scared that one day I might leave her (full disclosure: I had had a big argument with my wife a few weeks earlier where I gave the "that's it! I'm leaving" speech). After letting my daughter talk, and letting her shed a tear or two, she felt better and went to bed. The ghost was gone. If a ghost is bothering your child, don't try to trick them into chasing ghosts away. Take five minutes out of your busy schedule (yeah, you'll have to turn off the tv) to simply ask... "so what's REALLY bothering you?"
Edit: I understand that some kids won't talk as easily as my daughter does. Trying to get a word out of my son about his feelings is close to impossible. But when your kid tells you that something is scaring them, that's probably a good time to have a chat. Try to think about anything that happened in their environment in the last week or so that upset them. You'll probably find the tip of the iceberg there. If you get nothing but silence, default to the "fear is a necessary human emotion that helps us survive" speech. That's always worked for me. If anything it will probably bore them to sleep. :-)
