If you choose to retain anonymous posting, then, well, welcome to the wonderful world of Mankind. While most humans are civilized, honest and polite, there is always one user who thinks exposing his lack of education is a smart thing to do. The best you could do, then, is to patiently clean up (possibly proactively, by enforcing pre-publication control of all posts by trusted moderators) until the perpetrator loses interest or reaches the age of 14, whichever comes first. This may take a few weeks or months.