You can delete categories one at a time using the I/C Categories screen (the same screen you use to add categories to Inventory Control).
Note: You cannot delete a category if any items are assigned to it. You must first assign those items to another category or delete them.
Before trying to delete a category, you can set the category status to Inactive to ensure that no additional items are assigned to it.
You can then wait until the end of the fiscal or calendar year to delete it (depending on the method you use to accumulate item statistics).
You use the I/C Categories screen to make categories inactive.