Me, personally?

I'd ask them what could be gained by such a conspiracy?

If they brought up NASA I'd explain in no uncertain terms that NASA didn't exist a hundred years ago and yet people still knew.... Not believed, but KNEW that the earth was a globe.

Even the ancient greeks worked out the circumference of the earth over two thousand years ago. Now, in twenty first century we have people who refuse to believe anything scientists tell them.

Discard science? That means no medicine, no technology, no civilisation beyond a few others like them who herd goat and live in mud huts or tents.

I'd tell them to stop being hypocrites. They're quite happy to accept the fruits of science if it benefits them and then chuck everything else back in scientists faces claiming they're all part of the conspiracy.

Frankly, these idiots are an insult to modernity and humanity itself.