You can simply over-write and delete the HelpDesk category name. It will no longer appear on the HelpDesk view. The Complaints lodged earlier with this Category will still show. You can either continue them as-is, or change the Category.
Step 1: Go to Admin >> HelpDesk Tracker >> Set Up
Step 2: Just overwrite (or) delete the category you wish to modify.
Step 3: Click Save Changes.