Assuming I'm reading this question right, do you mean like the mother in the Stephen King classic "Carrie"? It could be a form of obsessive compulsive disorder. If it wasn't religion, they'd go overboard obsessing about something else. Short of committing them to the mental ward of a hospital there's not much you can do. Like many other problems, they have to realize they even have a problem before they can even start to deal with it.