Thank them for their time, sincerely, making sure the beginning of the statement acknowledges the value of the current thread of thought ("that's absolutely fascinating...and thank you for sharing that with me") and make sure your tone of voice descends at the end of the sentence; if they respond with confusion at this abrupt ending (it may appear so to them) let them know why you must go now or soon.
If your reason is impolite ("you're a boring jackass") you may wish to omit what you specifically think of them (the reasons why you think they are a jackass may have less to do with them and more to do with you and how you see the world subjectively, it's something that needs to be checked out at some point) and simply indicate that you are in disagreement with them and that you lack the time and energy to properly present your position and that you may or may not get back to them later.
Works 5/6 of the time.
This overlaps something I was wondering-- whether there are subtle clues you can give that the conversation is winding down.
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)