With people like that I just keep things very cheery and on-surface. "Thanks again for sharing your observations! I and our project team continue to appreciate your constructive suggestions and will take them under consideration." Some crap like that. (It's actually not quite a lie, since any genuinely constructive suggestions would be worthy of consideration.) That should put a stop to it, since the way you describe your work relationship with this person doesn't suggest to me that they are in a position to follow up with requests for status updates. In any event, my status update would continue to be "we're taking it under consideration; thanks again for kindly offering your feedback!" Fake smile; end scene.