Either way, they were obviously born with the emotional sensitivity of a brussels sprout, and a mind closed tighter than scrooge's wallet. Rise above it, hon. When you indulge in a verbal slanging match with this person, you descend to their level, which is obviously the intention on their part. If anything, the only thing they're deserving of is your pity, that such a state of mind is all they'll ever know. Just smile sweetly, tell them they're entitled to their opinion, and that one day you hope they grow up.
Whatever you do,
don't tell them that they're acting like a cretinous lump of genetically engineered toe fungus, or that they're nothing more than a fetid pustule on the unwiped arse of humanity.