You can hook into the load-edit.php
action to modify how wp-admin lists pages. Add this to your functions.php:
<?php
add_action( 'load-edit.php', 'namespace_modify_admin_list' );
function namespace_modify_admin_list() {
global $typenow;
if ( 'post' !== $typenow )
return;
add_action( 'pre_get_posts', 'namespace_modify_admin_list_posts' );
}
function namespace_modify_admin_list_posts( $query ) {
/**
* Modify query here to change how posts are listed, using
* the set method of the query object.
*
* Example:
* $query->set( 'posts_per_page', 10 );
*/
}
?>
May I ask why would you want to hide posts that have more than one category? Is this for access-control or filtering purposes?