When we speak of "forbidden" or "proscribed" we must also ask "by whom?" Who is doing the forbidding/proscribing? Theists get in a big panic because atheism seems to be removing the ultimate forbidder/proscriber and in a sense that is true..., but it is only true of a vision of morality that requires some... ultimate forbidder/proscriber.
There are other visions of morality that do have those who do forbid/proscribe - indeed offer most, if not all, of the definitive qualities associated with morality. Secular morality sees morality as a human social enterprise, of humans, by humans and for humans. So, when we ask, "Forbidden by whom?" those who posit secular morality, without god, have a ready answer: everyone else.
So, no, atheism is not an excuse to do the forbidden. It is a reason to become more sophisticated about one's understanding of morality.
When someone asks you, "Was it an AK-47 or a MP5K you used to shoot the victim?" it is perfectly legitimate to say, "Your question is a leading question. I shot no one with anything, much less with one of two particular weapons."
When someone asks "What is the ultimate source of morality without god?" it is perfectly legitimate to say, "Your question is a leading question. It assumes morality must have an ultimate source. I can account for all the necessary features of morality without an ultimate source."
Morality is a tool for helping people live together in a social context. In many ways religion is anti-moral, since it relies on conflict against "the other" to polarize its populace into not living with others or sharing a social context with them.