Au Contraire. Atheism can and should reject the whole enchillada without taking upon itself the dubious duty of demonstrating that it is even possible to make a clear, unambiguous concept of deity.
It falls upon those who claim that there is a need to accept a concept to show that it can be defined in a meaningful way.
In order to reject something you need a clear definition of what it is. An atheistic position contradicts an ignostic one.