By claiming as such, they will claim that God dealt with barbaric tribes with barbaric punishments. If they claim something so illogical, then I think the dawah has been done and there is no need for anything further to that (since how can an illogical person be convinced of anything he does not want to believe?). It answers the question of the OP I think (i.e how to answer them) but convincing them is not in our hands. Allah guides whom He wills and leaves astray whom He wills.