Once you are introduced to an idea you cannot stay neutral about it. You can brush it off as ridiculous, ponder its possibility, accept it, reject it, or do something in between. But, you cannot return to a lack of belief position if lack of belief is defined as a non-intellectual commitment or non-action concerning it. Complete neutrality about a concept is impossible since all concepts have an effect upon the hearer and illicit a response whether it be emotional and/or intellectual.
Christians' objections to defining atheism as simply the absence of belief in gods can extend to blatant misrepresentation of what should be a simple concept in order to divert attention from what atheism really is. In the case of this myth, we find that people will inexplicably distort the simple nature of "not believing the truth of a proposition" in order to pretend that it's not possible to merely lack belief in the truth of someone's claims. This makes the myth easy to refute.
To begin with, it is true that exposure to a concept will have some sort of effect on a person — even if that effect is pretty minor and perhaps even unconscious. If we go so far as to discuss and ponder a concept, then we will surely have some sort of reaction to it. Our reactions might be intellectual, emotional, or both.
None of this, however, prevents us from maintaining some sort of "neutrality" on the subject. Thus we have the first error in the myth: being affected by an idea is not the same as not being neutral on an idea. Perhaps it could be argued that 100% perfect and complete neutrality is impossible, but only because we humans aren't prefect at anything. Excepting such an extreme possibility, there doesn't appear to be any good reason to assert that we cannot stay relatively neutral about an idea we have been exposed to.
The next step which the myth attempts to make is to equate neutrality with a lack of belief, claiming that a "lack of belief" in something is the same as non-intellectual commitment, non-action, and thus complete neutrality. This is a rather complex attempt to describe something quite simple: if someone makes a claim, and I don't accept that claim as true, then I lack belief in the truth of the claim — even if I don't take the extra step of asserting that the claim is false. There is nothing here about non-intellectual commitment, non-action, or neutrality.
Given some claim which I don't believe, but also don't describe as false, there are many possible steps which might be open to me. I might think that the truth or falsehood of the claim is so important that I commit myself to a detailed investigation to learn as much as I can. I might think that the truth or falsehood of the claim would be very dangerous, and so commit myself to take significant action to try to prevent possible harm. I might find the claim to be too vague or incoherent to make sense of. I might find the claim to be so trivial that I just don't care if it's true or not.
An important question raised here is why people would go so far in redefining such a simple concept like "lack of belief." I am inclined to think that Christians who repeat this myth haven't taken much time to think about the topic and, more importantly, haven't taken the time to talk to any atheists or philosophers about it. If they had, their misconceptions would be cleared up rather quickly; then again, that would entail admitting that one might be mistaken about atheists and what they believe. How often have you found Christians and other religious theists admit such a thing?