Is atheism a belief?

[Bucky Ball]

That's the rub, Bel...it doesn't matter.  He's an atheist either way.  Not all atheists are atheists for the same reason..or even any discernible reason whatsoever.

If they are adult atheists who reject claims for no reason whatsoever, they are just prejudiced nonthinkers, and I concede that a lot of people may be this way. I hope no one like that is posting here. Or they may be atheists for bad reasons, rejecting claims with logical fallacies like, "I wouldn't like it if it were true, therefore I reject it." 

No. 
That assumes the default valid position is theism. 
Thesim is a very specific set of claims that requires no response, just like the existence of pink sparkly unicorns requires no thought or response. 
There is no "prejudice" involved in the dismissal of belief claims of any of the thousands of gods humans have cooked up.