What you have to do is be both firm and consistent in returning to the subject at hand. Firm doesn't have to be loud, it is more about the attitude with which you say things.
A child can tell from tone of voice when a parent is serious and so can these guys. You can observe this at work in any riding stable, some little girls are kicking as hard as they can and the horse is ignoring them. Others just need a slight touch to get the horse to behave because they have learned to project their intent.
If you are tentative, they will ignore you; if you come across as meaning it when you talk, they will pay more attention. This is something you can practice (first borrow a horse... no not really although it will help a lot if you have trouble with assertiveness). Insted of the horse, stand in front of a mirror and practice the phrases below (and in other answers) in different tones of voice until you get one that sounds firm. Then practice, practice, practice until it always sounds firm.
In practicing, it helps to exaggerate, if you are tentative, purposely trying to be nasty may just move you far enough away from tentative to be assertive. Exaggeration is the natural way we learn things. You are at point A and want to get to point B but can't seem to get there, then try to get to point C which is beyond point B. Sometimes the problme is that point A and point B are farther apart than you think. So you are changing enough to get to Point A.2 but not to Point B. Get a friend to help you out with this after you practice with the mirror for awhile. You want to know how you are perceived not what you think you did.
So what you do is interrupt them every single time they get off track. Her are some things you can say:
- We can discuss that if there is any time left over after we get
through the agenda.
- That is an interesting idea, why don't you write me up a cost-benefit
analysis. I will expect to see it by Friday. Now back to
- Let's get back on topic. Harry, what do you think about... (Make sure
you redirect to someone other than the two who are off topic.)
If they persist, then you need to interrupt sooner and more firmly. Don't let them get more than a sentence out on the other topic. If they still persist, you can tell them they are wasting everyone else's time and to stay on topic. You are the lead, these people are not your friends, they are your subordinates. If they can't get a clue after two or three redirections, you need to take them aside after the meeting and tell them their behavior is not acceptable and to clean up their act.