Atheism is an absence of belief, not a belief in absence.
You know that those are logically identical, right?
A simplified example: "John doesn't believe God exists" is identical to "John believes God does not exist" as they both expresses, unambiguously, John's beliefs about the existence of God. That is, to the question "Does John believe God exists" both statements evaluate the same way: "no".
I understand why you want to say ridiculous things like that. It may even be effective if your opponent is a moron. But it's complete nonsense. If you care, at all, about reason and logic (as many internet Atheists claim) you shouldn't abandon it so readily, regardless of your motivation.
Now that that's out of the way, this is what you *actually* want to do is differentiate between gnostic and agnostic atheism. Take a look around here, or (if you can stomach it) JREF and you'll find quite a few gnostic atheists -- those that believe, with impressive conviction, that no gods exist and assert knowledge to that effect. The more sensible agnostic atheist, while lacking a belief in god, claims no knowledge of that fact, they merely don't believe in the existence of any god.
The crux of that is 'belief' Belief is an interesting thing, as belief is not subject to the will. That is, you can't simply choose to believe or not believe in God or anything else. For example, you can't, no matter how hard you try, force yourself to disbelieve in the existence of Tom Cruise. Equally, it's impossible for you, through an act of will, to believe in Santa Clause.