Conversations wander. It's what they do. I wouldn't dream of trying to stop that, I don't even think it's possible.
I don't think it's necessary or good to try to control that. The ebb and flow of conversations are what make a lot of these threads interesting and fun. There's probably a point at which a mod needs to step in and bring it back (and they do), but for the most part that's the kind of thing that makes this forum freer than most.
I do find it amusing, however when people gripe about one opinion on a topic that they disagree with, but keep coming to threads where that topic is being discussed. A little self-discipline is in order.
And then there are those who come here and seem to absolutely hate the place and the majority of its posters.
I find that very odd, and a touch masochistic. It's not that you can't find any place else to post on the interweb if you hate it here.
Just weird.