Hide tag and category boxes from the post editor

Using remove metabox function you can do this. Simply put this inside your themes functions.php file at very end.

NOTE – unwrap <?php ?> if necessary.

<?php 
function wpse60590_remove_metaboxes() {
    remove_meta_box( 'categorydiv' , 'post' , 'normal' ); 
    remove_meta_box( 'tagsdiv-post_tag' , 'post' , 'normal' ); 

}
add_action( 'admin_menu' , 'wpse60590_remove_metaboxes' );
?>