How do you not answer politely but firmly?

Don't say you cannot hold up. Do not say anything you don't want to say. You are not an ombudsman, a charity or answering machine. Please indicate in a correct and friendly way that you do not want to answer. There is no obligation to answer. It's not cross-examination, you are not in Guantanamo. Also, do not surmise what the correct answer might be. If you don't know, you don't know. Say it.