Categories can be deleted at any time. One thing to keep in mind is that when deleting a category not all related expirations are deleted.
This is to allow the final Expirations within that category to finish out their remaining time period (if any) for auditing purposes.
Delete Any Remaining Expirations
If you'd like to delete the expirations for that specific category, first click on Expirations on the top menu and then do a search of all expirations with that specific category.
Select all the expirations from the search results (screenshot below) and from the Bulk Edit menu, click on the trash icon on the bottom. This action will delete all expirations remaining for that particular Category.
Delete the Category
To delete the Category, click on Your Name drop-down menu, then on Settings where you can find the "Categories" link under "Manage"
From the category list, click on the black arrow pointing down and from this drop-down menu, click on Delete.