You have to edit the page directly, and remove the category from the list of categories on the side-bar.
When in editing mode, check the right side of the editor and you will see a list of categories. If you hover over a category, you will see two icons pop up: a pencil for editing the name of the category and a trash bin icon for deleting the category.
If you are using an iPad or another mobile device, the icons may not always work, but I know of a roundabout way to force the categories into source code.
How do you do that Gsi, cuz I hate when it doesn't work.
Under "Preferences" -> "Editing", there's an option called " Prompt me if I leave a blank edit summary". Apparently, if you publish an edit with a blank summary while that thing is on, the page reloads with categories forced into source. If you delete the code for the category, you'll remove the category from the page.
Of course, that was not the reason I had that thing on, but I noticed that it had another use. Though I wouldn't recommend using this method for editing sections, as there appears to be a bug with it on the section editor.