If a student uses the phrase: "the scriptures say" or "the prophets say', consider if you can take the opportunity to teach students the importance of being able to find and read a scripture during a discussion. It's best to have scriptures memorized, but if not, teach students how to find scriptures quickly in a pinch. This skill is an absolute MUST for future missionaries.
Ask the student if they can show the class where the prophet or scripture is that says that thing. Then help them find it, or gently correct their mistake.
Be very careful using this method with adults. I would never use it with adults if the verse or statement is being misquoted, because the correction could seem very aggressive. Unless I were absolutely certain that the scripture being quoted was correct and we could find it relatively easily, I would not use this method with adults.