Not at all. Atheism is the recognition that the available evidence is insufficient to prove the existence of deities. That naturally leads to a lack of belief in deities. It does not cut off investigation is some evidence does appear.
In fact, atheism follows naturally from the recognition that all phenomena we have seen can be explained by natural explanations. So, at the very least, it isn't unreasonable to test such explanations before any supernatural ones. And that is without considering whether supernatural explanations are explanations at all (being untestable).
That sounds like the definition of free thought not atheism.