One of the many frustrating falsehoods we hear from the religious is that "atheists don't believe in anything". Frankly, this is another one of those things that make it nearly impossible to respect the intelligence of believers. A person who doesn't believe in anything is called a "nihilist". An atheist on the other hand is someone who doesn't believe in one particular type of thing: gods. So, first and foremost, your response should be to point this out:
"A person who doesn't believe in anything is called a 'nihilist' not an atheist. Do you not know that word? You do know the difference, don't you?"
Then explain the difference.
This is another case where believers deliberately obfuscate the difference between the various definitions applicable to a word. Belief, like many other words, can mean many things. It can mean an opinion or conviction, confidence in the truth of something without proof, or simply confidence in a person, thing, or idea. But, what the religious are disingenuously doing in this instance is twisting our unwillingness to believe without evidence into a lack of belief in all things, which of course includes morals and other values. This is just an example of bigotry out in the open--just another way of calling atheists evil.
Call them on this point as well.
"You are deliberately confusing the different definitions of 'belief' in order to express and justify your bigotry."
You can add:
"You're trying to insult us, but you're just making yourself look bad because now everyone knows that you don't know the difference between an atheist and a nihilist."