The concept of off/on topic in threads is really really difficult. The solution I've been most successful with (and I'm pre-Internet chatforum moderator old) is to have the original poster decide when the discussion that follows is on or off topic.

Else you end up in situations where the thread starter is fully ok with the discussion and a moderator starts deleting posts.

(In this case it should be somewhat easy for moderators to see if it's the thread starter that Reports to Moderator with "off topic" as reason)