There's not really an easy way without editing theme files, but you can use CSS to hide the subcategories manually:
#posted-in-categories [value="13"], #posted-in-categories [value="6"], #posted-in-categories [value="5"], #posted-in-categories [value="17"], #posted-in-categories [value="18"] { display: none; }The number in quotes is the category ID, which you can find in the ACP.