This "creating a competing emotion" workscan work really well.
What I'm trying to say is that telling them the truth (ghosts not real) is necessary but not sufficient. The child needs to work through the emotional component of their fear, which doesn't just go away when they realise that the fear is not founded in fact.
One way to help them work through that emotion is to tell stories together with your child, in which:
- positive emotions and associations (such as laughter and parental closeness and support) replace negative ones,
- the child exerts control and dominance over the fictional object of their fear, e.g. by scaring it or calling it rude names.