@shinyidol I agree, having a proper audit log is a good idea, and ideally should be built into core.
Our logging system could use a revamping, however...
I hope create a user, and the user only delete the topics in one category but not all categories. Would you tell me how to do this please?
Its possible to assign a group the moderating power on just one category, However be aware that this will also include other moderation powers such as Forking, Closing and Moving topics.