In his book Unweaving The Rainbow, Richard Dawkins boasts (boasts!) that he told a six-year-old that Father Christmas doesn't exist. His logic was purely scientific - there wouldn't be time for Santa to reach the homes of all the good children in the world in one night.
A few years ago I lampooned this idea with a similarly scientific rebuttal: Santa can do everything he claims provided he is a macroscopic quantum object. In this way he can be in as many places as he likes, provided that he remains extremely cold, and nobody is watching. Not only does this trounce Dawkins' objections, it also works better as a scientific hypothesis, because it accounts for more of the evidence: we now know why Santa is traditionally associated with cold places, and why he does his work while everyone is asleep.
Henry Gee, in this column from nature.com, points out the fundamental flaw in trying to demolish faith scientifically. He describes himself as one who believes in God and subscribes to evolution, the former being personal and the latter a matter of science. One is subject to the methods of science; the other is not.