Hi John or Jane Doe,
I’m going to respectfully decline to sign the NDA. I assure you that I have not shared the idea and that I am not pursing it myself. I have neither the time nor your passion for it. Nobody else does either.
I suspect this is not your primary concern and will speculate that you are more concerned about public disclosure as part of the patent process. Allow me to suggest an alternate course of action:
Instead of investing the limited time, money and mental energy you have in pursing a patent, put all of your effort into building the actual product and building a customer base. Every single bit of it. That will mean talking to as many people as possible about your idea, sharing it as widely as possible, getting as much feedback as possible and as quickly as possible, and forgoing the use of an NDA.
There is a minor risk your idea will be stolen. There is a significant risk it will never get built. There is a more significant risk it will get built but no one will use it. Focus on the second two.