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.
Think of CC-By-SA as an agreement between an author and those who want to use the content. Normally, under the DMCA, you either need the express permission of the copyright holder to: share it in full, create derivatives, or commercialize it or you need to use the work in a way that constitutes Fair Use.
Both of these things can be absolutely prohibitive for both the author of the story and the creators who want to use it. Because of this the CC-By-SA license comes into play. It is an additional agreement between the author and the creators that allows the creators to use the work how they wish as long as they meet some basic requirements.