As the answers by wimh and Da;e M correctly point out, section 7(b) of the CC-BY-SA 3.0 license declares the license perpetual , and provides no way for the copyright owner to cancel it, unless the person using the work violates the terms of the license. That is one reasons why a number of websites that depend on user contributions, including Stack Exchange and Wikipedia, use a version of the CC-BY-SA license.
Termination Rights Under 17 USC 203
It is true that if the license is granted under US law, the provisions of 17 USC 203 permit the author to cancel any and all licenses, grants, and transfers, for any reason or none. But that can only be done during a five-year period starting 35 years after the work is created, or in some cases 35 years after the work is first published, and the author must give two years notice. But this right of cancellation cannot be contracted away in advance. Still to use it, one would have to wait fore the end of the 35-year period.
Termination Rights Under the CC-BY-DA license
On a more practical note, if the site operator violates any of the limitations in section 4 of the license, the licensee (recipient, in this case the web site operator) will have his or her the license terminate. This includes the obligation of the licensee to preserve the original copyright notice, if any; to provide proper attribution; to provide a copy of or URI for the license text; to remove any credit line on request; and to provide proper credit to the person granting the license, in the absence of a request to remove credit. A public post falsely denying the authorship of a post would seem to be a failure to grant proper credit, and would terminate the license.
Effect of License Termination
If the license is terminated, further display becomes copyright infringement, just as if no license had even been granted. In this case cease & desist letters, DMCA takedown notices, and copyright infringement suits may be used, although in the US a work must be registered before any infringement suit can be brought.
Data Protection Rights
Also, if the site operator is locates in the EU pr UK, or you are, so that the GDPR applies, or you are located in California so that the CCPA applies, or if any other Data Protection law with similar provisions applies, the post may be considered to be "Personal Data" (PD) because it is associated with a specific identifiable natural person (the author), If so, the author may have the right to have the post removed undern such a data protection law.