I don't know .htaccess but I know it'd take what I believe is called the rewrite module or something along that line, but it can be done in php through using
PHP Code:
$cat = $_GET['category'];
$id = $_GET['Itemid'];
if($id != '' || $cat != ''){
header("Location:index.php");
}
That SHOULD work, and I'm sure someone will be along shortly to show you the .htaccess.