Let me address some gross misunderstandings here.
First, when you create a work you automatically have a copyright on it. You made it, it is yours, that's it. To be clear, copyright is given upon creation not when you post it.
Second, CC-By-SA does not void your copyright. When you license a story under CC-By-SA you are not giving up your copyright, it is still yours. CC-By-SA can be thought of as extending what is allowed under Fair Use. It makes it clear that you are able to remix and share it but only as long as you give credit to the copyright holder, in this case the author, and that any derivatives are to be licensed under CC-By-SA.
Third, you cannot void copyright or CC-By-SA. When you, the author, post a story here you acknowledge and agree to release it under the CC-By-SA license. If you then to go on to post it on to another site without following the Share-Alike and Attribution you are breaking the terms of the license. While you shouldn't do this you are, in the end, the one policing the use of your content.
Now, if another person we're to share it in a way that violates the terms of the license you are free to use the recourse provided by the Digital Millennium Copyright Act to remedy it. But their misuse of your content doesn't void the license.
Also worth noting, you cannot revoke the CC-By-SA license.
Once you post it under this license that content is irrevocably CC-By-SA.