You're allowed to be as closed-minded as you wish, but it is not scientific to completely and smugly dismiss any possibility you do not possess the data to to completely exclude.
It would seem that your understanding of science is very limited. You don't need to completely exclude anything in science. In fact, you
can never completely exclude anything in science. That can only be done in mathematics and logic.
In science we do not completely exclude anything, we accept probable theories. We ask, does this theory have enough evidence to be considered credible? If the answer is yes, we accept that theory until something comes along to show that the evidence better supports a different theory, or some evidence comes along that precludes much of the supporting evidence from the old theory. Scientists don’t go around believing in things for no reason. That is not scientific.
You can rank possibilities according to probability, of course, but being completely closed-minded without proof is to be just as silly and superstitious as any religious fool, yo.
Now this makes more sense, but it undermines what you are trying to say. That is what the vast majority of what atheists do. They use logic and evidence to rank the probability of religious claims. It turns out that almost all those claims have a really, really low probability. When the probability is that low it is safe to say it is not true. With out those claims, there is nothing at all to even try to believe in. It is not that we are not open to possibilities. It is that we have looked at those possibilities and decided they are not at all probable. Come up with more possibilities and we will evaluate them as well.
Alternatively one could entertain the question of why one seeks to dismiss anything in the first place.
Because I have limited time and resources and have to find some way to allocate them.
If I have to attend every possible church, make every possible sacrifice, and stick to every possible restriction, then I will not have time to do anything else.
a·the·ism
   /ˈeɪθiˌɪzəm/ Show Spelled[ey-thee-iz-uhm] Show IPA
–noun
1. the doctrine or belief that there is no god.
2. disbelief in the existence of a supreme being or beings.
http://dictionary.reference.com/browse/atheism
That feels like it was written by a theist as an attack on atheism. I have met very, very, very few atheists that would agree to that definition. It is like if Dictionary.com published a definition of Christianity that stated:
1. A doctrine that a magic man existed two thousand years ago who told people that cannibalism was the only way to live forever.
2. The belief in magic man cannibalism.
So, my question to those that agree with this definition of atheism: what do you call the 99% of atheists that do not believe in a god, but does not have a positive believe in no god.