I think the possibility that an off-track conversation indicates that your interlocutor has reached the end of his knowledge on a subject should be seriously considered.

Telling people to stuff it is probably not going to end well. I was raised in a small talk culture, in the Great Plains/South border region, and the way you handle this there is "oh, but back to what you/I said earlier" and then you act like the topic hadn't ever stopped. It seems to work all the time, though I've never been of a mind to test it rigorously.