Arguing with him just provides him with incentive to continue.
That's it, quite simply. Leave these people alone.
It's not just nutcases either. I recently was accused of 'cognitive dissonance' on Reddit for saying that it is crazy to believe in Allah (et al.) but not crazy to believe that there is complex extra-terrestrial life. (NOTE: it also wouldn't be crazy to believe the opposite. I personally am undecided, but believe it is very possible) When I carefully explained that one belief is founded on Bernoulli's rule, the Drake equation, possible theories of abiogenesis etc., theists still said that it was no better than believing in god.
They are determined to:
1) bring scientifically founded 'belief' (really, from an 'armchair epistemological' view, 'knowledge) down to the same level as blind faith
2) argue that, since it is just another 'faith', they can and should reject it along with every other religion.
4) send everyone back to the dark ages!