Not sure if you looked at any of the settings or not?
So I just tried via some tests…
If I go to any category to post and change the category to “Select a category…” and try to post I am told I must select a category.
Thus, a user could not currently get a post into the uncategorised category anyway.
If I go into the uncategorised category and try to make a post I am advised to select a cateogry, which I cannot do as the drop down menu for categories is empty - and mandatory.
So, so far having a category called uncategorised seems fairly redundant.
One thing I am unable to test is what hapoens if, as an Admin, you choose to delete a category, are you given any choices with regards to the topics contained within. Would they for example all be moved to the uncategorised category, therefore creating a purpose for it through the forums own default behaviour, or, are there no such choices and all the posts just get deleted?