Maybe just appeal to people's better nature? Surely in a civilised community like this one it ought to be possible to ask people to do their own moderating in this respect?
Imagine you’re at a crowded dinner table where, meal over, there’s a lot of chat. You make an idle remark to somebody sitting diagonally oppostie you, they respond, and in no time you have a serous conversation going. But you’re having to shout in order to hear each other, so to make it easier to talk to the other person – and not to impose your conversation on those around you – you move over quietly to sit alongside them.
Virtual equivalent, if you realise that you’re having an interesting debate but it is off topic, is to say: “this is interesting but maybe not of interest to people who have tuned in because of the title, so I’ll continue it in a fresh thread”.
If members won't do this, then, Paul, I appreciate this is a difficult call. Personally I could have done with some hands-on moderating in the recent animal posts. Some poor woman asks for a recommendation to a vet, and suddenly we've got links to horror videos?