To change a Category from Active to Inactive, hover your mouse over the Manage button in the header menu.
A drop down list will appear.
By selecting the option "Categories", a new page will open up with a list of all your Company's Categories.
Select the Category which you wish to make Inactive.
Notice, that on the right hand side you will see the word "Active" with a drop down arrow next to it. Select the drop down arrow, and then click the Inactive option.
The Category is now Inactive.