I can understand your frustration a little bit, however -
I feel it is the nature of conversations, be they online or face to face, to wander on and off topic. Sometimes these meanderings as still of interest, sometimes they aren't.
If I find a topic that I am following wanders off into an area I'm not interested in I just stop reading it.
I think moderation, other than for offensive or provocative content, would be too oppressive and heavy-handed.
Just my own opinion of course.