An absence of belief is exactly what is required. If somebody asks if you believe in big foot, your response is an absence of belief - which is a "no". That's exactly what atheism means.
Atheism is an absence of belief in the existence of deities. If somebody asks me if I believe in bigfoot I can have three answers:
1. I believe in the existence of bigfoot, which would be the equivalent of "theism".
2. I have an absence of belief in the existence of bigfoot, which would be the equivalent of "atheism".
3. I actively disbelieve in the existence of bigfoot, which would be the equivalent of "hard/strong atheism".
1. I believe. Bigfoot may exist or not, but I believe he does.
2. I have an absence of belief but not a presence of disbelief. Bigfoot may exist or he may not. I have no beliefs on the subject.
3. I disbelieve. Bigfoot may exist or not, but I believe he doesn't.
Just think of an elevator standing still on one floor. You can decide to go up (theism), you can just stay where you are (atheism, absence of upward movement) or you can decide to go down. (Hard/strong atheism).