Couple of things...
Make sure that as well as giving registered users the tiki_p_read_article permission, that you've taken it away from anonymous users too.
Also, it may be the case that the anonymous users still have tiki_p_topic_read permission, so they can still see the topic (and possibly the articles in it) but they might not actually be able to read the individual articles.
Generally, for that particular topic, just take away all anonymous privileges.
Finally, try clearing your tiki cache (tiki-admin_system.php)
I know this works because I've done the very same thing, it's just a case of getting all the permissions right.