According to the Quran, Here is what Mohammad said his fellow arabs said to him.
Surah 15:6 And they say: O thou unto whom the Reminder is revealed, lo! thou art indeed a madman!
Surah 44:14 And they had turned away from him and said: One taught (by others), a madman ?
Surah 37:36 And said: Shall we forsake our gods for a mad poet ?
Surah 8:31 And when Our revelations are recited unto them they say: We have heard. If we wish we can speak the like of this. Lo! this is naught but fables of the men of old. So how can any present day Muslim claim to be surprised or shocked when they hear such things said now a days ?
Maybe the moderate reply would be with
Surah 15:3 Let unbelievers eat and enjoy life, and let (false) hope beguile them. They will come to know! 15:43 And lo! for all such, hell will be the promised place.
Why not just shrug and let it go? "It is your risk, we just want to warn you."