<?php

global $wp_query;

// If the category name is set for this page...
// (which it will be when viewing a category on the category.php template)
$category_name = $wp_query->get( 'category_name' );
if ( isset( $category_name ) && ! empty( $category_name ) ) {
    // Now we have the category name, but we want more so we're going
    // to get the full category details using the slug name.
    $category_all = get_category_by_slug( $category_name );

    // Now that we have all of the category details, we can safely
    // extract the category ID.
    $category_id = $category_all->cat_ID;

    // Using the category ID we just obtained, we can get all the children
    // of this category (returned in an array).
    $category_children = get_term_children( $category_id, 'category' );

    // And now we just need to pass this array of children to exclude
    // into $wp_query. We'll use query_posts() for this. Don't forget
    // to merge in all of the previous arguments.
    query_posts( array_merge( $wp_query->query, array(
        'category__not_in' => $category_children
    ) ) );
}

// Now run your WordPress loop as normal and notice that posts from
// child categories no longer appear when viewing a parent category.
if ( have_posts() ) : while ( have_posts() ) : the_post();

    the_title();
    the_content();
    // Other stuff you do

endwhile;
endif;

// Lastly, you may want to rewind your posts since we used query_posts().
rewind_posts();

?>