Nobody can entirely shut out the idea of One true God, the creator of the universe and everything contained within. There is no proof against God, none. You can argue this and that have science try to explain everything under the sun and beyond, but you cannot disprove God.
Why is it that atheists say they flat out deny the idea of God? Is it because we haven't proven God with science? Is it because they have lived a rough life or see the hurt and pain in the world and do not see how a loving God would allow it to happen?
God isn't just some theory men made up thousands of years ago, theories can be disproven, God cannot. Just like I cannot prove that God exists to somebody, an atheist cannot prove to me or themselves that He does not. There is nobody that can deny that there is a God, they can only announce their disbelief and reasons for not believing. In order to completely deny even the possibility of a God a person would need proof.
There is no proof in science, we only explain what happens and accept it as truth because there are no exceptions. We can explain and predict behavior, but we cannot explain why it happens. We know an apple falls to the ground when it falls out of a tree, we know there is a force that brings the apple to the ground, we call this force gravity and know that it exists. BUT what is gravity and why does it happen? I'm sure science explains the behavior in even greater detail, but yet they can not explain everything about anything. Why? There is a unknown force behind everything that happens, this force cannot be seen and this force cannot be explained by science.
So my answer to the OP is:
Yes, by definition, there is such a thing as atheists, but there is no real reason for anybody to completely shut of the idea of God because they have no proof to prove their disbelief rational. Just as unbelievers have no proof to prove Christians or any other theist irrational.
Maybe that's why a Psalm says that the fool has said that there is no God.