You say to the other person, loudly and firmly "hang on X, I haven't finished" then you carry on until you're done.
There's no magic to it, no reliance on a corporate directive, or gimmick like a talking stick or hacky sack. You just directly address the other persons behaviour.
I was in one particular meeting where I had some technical details I was trying to give (that were actually relevant and important). But each time I started trying to give them, I got interrupted by the marketing people. (One in particular. She was pretty loud, too.) I backed off, because I figured that there would be another point in the meeting where I could lay out the tech stuff.
But after the third time, when she interrupted, I just continued (louder than her), "As I was saying, the way the tech works is..."
She didn't interrupt me again. (Of course, I didn't say much more in the meeting once I said what I needed to...) I don't remember whether she interrupted anyone else after that.
While I upvoted your comment for it's directness, and agree that it can work for infrequent instances, I've seen whole cultures where everyone is interrupting each other. It's harder to apply when things get truly bad.
Incidentally, that was in the bay, and I've had better experiences elsewhere - purely anecdotal.
This is the right way. People know interrupting is rude and if you call them out immediately you can just continue. Adding some gesture helps too, like raising the pointing finger :)
There's no magic to it, no reliance on a corporate directive, or gimmick like a talking stick or hacky sack. You just directly address the other persons behaviour.