I'm with Lardman in that I don't think additional moderating is necessary and gentle reminders.. or even just taking the initiative and taking the topic back to its main point again to get it back on track is the most appropriate thing to do. Also, what did you think of that latest Batman movie?

Please don't hurt me.. not the face.. NOT THE FACE.

In all seriousness, though.. I find that ignoring the veering and getting the discussion back on track with a reminder about the off-topic chat is usually enough and sometimes people don't even realize that they're going off-topic.