Love, love, love Connie's responses. I'm stealing them! I have a variety of responses that I've used.
1) I can raise one eyebrow, so I like to do the stop, pause, and stare with the eyebrow raised. Because I have a fairly Cro-Magnon brow, so most people stop what they're saying and look slightly scared. The eyebrow is the method I employ the most, probably because I'm inordinately proud of my ability to raise just one!
2) For someone who's persistent, is loaded with all those facts, and who just keeps talking. I just keep saying, "Yes, yes, you're right. Yes, of course you're right." Over and over again, like a loop. Somehow when I acknowledge a person's rightness, it takes the fun out of the argument.
3) When it's someone close to me, the subject is brought repeatedly, and I'm seriously annoyed I will say, "My body. My choice." The first time I said it was to my MIL when, after this year's Goofy, she started in on DOOD again. She said she really thought it was time for him to stop running marathons (at a minimum) because of his knee, all those studies, etc. We'd we'd been talking about his desire to do an ultra. So I said, "It's DOOD's body. It's his choice." And she was offended, saying she NEVER told her children what to do, that she supported him no matter what, and that she was just expressing her opinion.

But the phrase worked--she's not told him to stop running again. (We'll see what this weekend holds.) And I've found it works for me, too.
4) And then there's my so-called sense of humor. If it's a fairly innocuous comment, I might say something like, "Yeah, I tried to kill myself quickly by drinking an entire bottle of Draino, but that didn't work." Or "Yeah, I know, but my doctor tells me that running helps me curb my impulse to make fun of uninformed people." Or "Yeah, running might hurt my knees eventually, but I figure the mafia's going to get a crack at 'em first. I have many addictions that I'm gonna have to pay for.
Seriously, people are going to keep saying things like that over our running careers.

I don't really know why, don't really care. I just keep practicing my breathing exercising my breathing exercises and my mantra "all about me." As in, doing this is all about [taking care] of me and my needs.