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I have the impression that in order to delete someones posts, you have to go to his profile, find them and delete them.
It can be done only on purpose, unless the moderator has deleted complete threads.
In any way, the "moderators" have to reply to the following question:
Hi moderators, are deleting posts/threads and if yes, for what reason?
EDIT: Some people have deleted their own posts. It does not seem that there is any other issue.