Then I am also entitled to keep my details private; I am sure we can both agree on that. Duly noted, I shall not ask you for any more details regarding your former place of employment, I am only seeking recommendations as to what is a good place to work at and what IS NOT. No more, no less. If I offended you, I apologize profusely for that, it was not my intention.

Just the right response in the circumstances.You have said what was needed to be said politely, but more importantly firmly.