From the Creative Commons Full FAQ:
Creative Commons licenses are non-revocable. This means that you cannot stop someone, who has obtained your work under a Creative Commons license, from using the work according to that license. You can stop distributing your work under a Creative Commons license at any time you wish; but this will not withdraw any copies of your work that already exist under a Creative Commons license from circulation, be they verbatim copies, copies included in collective works and/or adaptations of your work. So you need to think carefully when choosing a Creative Commons license to make sure that you are happy for people to be using your work consistent with the terms of the license, even if you later stop distributing your work.
I did not know that.