Thanks for your reaction.
The spam posts are not from visitors. The posts are sometimes without a book-id and often without a rating. That is technically not possible when a normal visitor add a post. Often the posts are added three times (the same) and most of the spam posts are about sex. So, the posts are not from normal website visitors and I am afraid it has nothing to do with the popularity of the site. Do you have a solution pf suggestion for me? How can I protect my site against unwanted spam posts?
I added an example of some (decent) spam posts in my review table.