I'm afraid my response would not be as diplomatic as those suggested in the other answers.

This concerns last year undergraduate students.

You are dealing with adults. Moreover, your are dealing with adults who are likely paying tuition in order to attend your lectures. This isn't grade school; attendance is not compulsory, and they are free to leave if they feel your class is not worth their time.

For the first and second incidents, I would respond as suggested by academic:

"Excuse me, your classmate is speaking. Do you have something to contribute?"

Hopefully this puts an end to their disruption. If not, I would respond to subsequent offenses with stronger language:

"Excuse me, why are you here? If you don't feel like you can prevent yourself from disrupting our class, you are free to leave. No one will stop you. If you need this class for degree credits, consider dropping it and re-enrolling at such a time that you've matured to a point where you can contribute meaningfully to the discussion."

I realize this is more confrontational than most instructors are willing to be. That said, embarrassment can be a very effective means of making your point.

I'm afraid my response would not be as diplomatic as those suggested in the other answers.

This concerns last year undergraduate students.

You are dealing with adults. Moreover, you are dealing with adults who are likely paying tuition in order to attend your lectures. This isn't grade school; attendance is not compulsory, and they are free to leave if they feel your class is not worth their time.

For the first and second incidents, I would respond as suggested by academic:

"Excuse me, your classmate is speaking. Do you have something to contribute?"

Hopefully this puts an end to their disruption. If not, I would respond to subsequent offenses with stronger language:

"Excuse me, why are you here? If you don't feel like you can prevent yourself from disrupting our class, you are free to leave. No one will stop you. If you need this class for degree credits, consider dropping it and re-enrolling at such a time that you've matured to a point where you can contribute meaningfully to the discussion."

I realize this is more confrontational than most instructors are willing to be. That said, embarrassment can be a very effective means of making your point.