Give them a grade for it. Even if it's just a few points. I know it will throw off the presenter, but don't let it continue without immediate correction. I don't have this problem. My kids do a LOT of presenting, often in form of informative speech, maybe with visual posters or powerpoint presentation.
Nobody talks during it.
This is really a pet peeve of mine, but it's also valid from a management angle. This is harder. At the beginning of the year, it's always hard to get them to raise their hand when they have something to say. Within first few weeks, no problem. Hope this helps.