I can definitely sympathize. Weird strangers just seem to love me for some reason. When the conversation is inherently short (ATM line, passing someone in the grocery store, etc) you can usually smile, say something neutral like "wow, sounds like its been a rough day" or "yeah, that is sort of bizarre," then quickly move on. If you're stuck somewhere, like the bus, I would say basically nod along unless you can't in good conscience (they're saying some offensive, etc), in which case a short "well, I can't say that I agree" should suffice. If they persist, you should be able to tell them that you're just exhausted from a long day, distracted by other things, whatever, and aren't in the mood to debate.

If you know it's going to come up on your commute, bring a book and/or earplugs. I had some bizarre people try to talk to me oftentimes when I would wait for the bus, and headphones tend to do the trick.

On the lighter side, I heard a comedian say that when a stranger got mad at him and started an awkward confrontation, he just started a completely different argument. As in "the light is green, go!" "well, you should give me my jacket back!" "what?" "I said you could borrow it, not have it!" Maybe you can just start a random conversation of your own, on a completely different topic :)