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 Image 1

 

However you can edit any category by double clicking on it. You can also set an unused category to inactive (uncheck the Active box).

 

Make Unused Categories Inactive Image 2

 

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