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.
