To delete a category, select its name in the category tree in the Documents and Categories window, and press the 'Delete' key on your keyboard. Alternately, choose from the . The category, categories attached to it, and any memos and coding associated with them will be deleted from the project.
You should think very carefully before deleting a category. Once deleted, a category cannot be retrieved except by reverting a previous version of the project, so losing all your changes since the last save.