AMG has a list of default categories that cannot be deleted. They are listed here (depending on the modules for your software):

Make Unused Categories Inactive (Web) Image 1

However you can click edit next to the category name to edit it. You can also set an unused category to inactive (uncheck the Active box).

Make Unused Categories Inactive (Web) Image 2

If a category has been used at any time it cannot be deleted, nor set to inactive.