In seriousness, you need two things.
The easy thing is a clear and obvious signal that this needs to end, now. It could be a stock phrase. I knew a GM (not me) who had and used a 60 second hourglass for "wind it up". The signal doesn't matter as it's the easy thing.
The hard thing is enforcing it. When you use the rule, mean it. If they continue, game time passes. Bad guys move, bombs tick closer to exploding, things happen.
hmmm. Add a third, and possibly the hardest. Learn when and how to break. Sometimes all this is a clue that the group needs a break; maybe a short snack break, maybe a "see you next session" break, but a break. Sometimes it's just necessary, and forcing it anyway starts turning the game from fun to a chore.