I operate this way in two different methods:
i establish my index.php to only show the category i want.
this can be done more than one way:
a. just below (or after) "do not remove this line" add
now your code will only allow the display of category "x" on the index page.
this works, but can become a drag when you actually want to use the index to view other categories.
i worked around this by creating sub pages/folders with the rest of the cats so they were easily called up using my own navigation, or links.
b. just below (or after) "do not remove this line" add
This allows you to select multiple categories for display on the index, leaving out the onces you leave out.
and again, you can either alter your category output in the menu to display only what you want to see, or, navigate with custom navigation and or links.
an example of this is here:
notice the "about" links in the menu. they are intentionally excluded from the index, but accessible via an "about.php" where they display only the about category.
hope that's useful.