Can an Atheist argue someone out of faith?

[Tiberius]

Why does the theist have the burden of proof?

I dont believe Santa Clause exists because I have reasons no to.

You are just trying to get off the hook.

The burden of proof always starts out on the person(s) making the claim. See: Burden of Proof. The claim of theism is that "god exists", thus the burden of proof is on the theist to show that god exists. The burden of proof will only switch to atheists once a sufficient "proof" has been derived for the existence of God. So far, everyone is still arguing over whether various proofs are valid or not, or whether a proof for God is even possible.

A lot of theists say that God cannot be proved, and it requires some sort of faith in order to believe. That is all very well, but it doesn't shift the burden of proof; it is just an admission that the burden cannot be fulfilled, and that the atheist position is not impossible.