I have a student who constantly interrupts and butts in to conversations and questions. No matter what the topic of conversation or the question asked may be, he is an authority. For instance, whenever another student asks me a question, before I can even open my mouth, this boy butts in with an answer or with a "yeah, how come...." question of his own. Most of the time, his "yeah, how come..." is on a completely different topic! I get so frustrated with his interruptions that I sometimes forget that another student had a question.
I've talked to other teachers who are having the same problem with him. Is he just trying to get attention? Do you have any suggestions on how to deal with him?
Since this happens all of the time during regular events in class, you do need to discuss it with him and try to get to the root of it. It could be that he wants attention, or just that he has not learned self-control. First try discussing it with him privately. Treat it as something you can work on together. Work out a signal to stop him when he interrupts. If none of this works, have a parent conference.
I've come across this issue a few times in my teaching career, and it can be completely infuriating. In the past, I've given a participation grade each marking period (I usually count it as a quiz), and the kids start the marking period with a 100% for participation. If they are late, disrupt class, come unprepared, etc., I deduct points. It's worked pretty well, but it's a little inconvenient.
One tactic that you could sometimes employ: try breaking the class up into small discussion groups, and have them develop a series of questions re: the subject at hand. Once they return to the large group, have members direct their questions to one specific group or another (so group one asks group two questions, etc.). Only the called-upon members can answer questions. Your student will have already had a chance to share his knowledge (during the development of the questions), and the structure will keep him quiet until it is his group's turn to answer questions. If he breaks the structure, the other students will correct his behavior.
In addition to body language, another tactic you may choose to use is completely ignoring him (within reason) when his statements are not in line with your classroom procedures and expectations. If, for instance, your expectation is that all students will raise their hand to be recognized before speaking, he is violating your classroom protocols, and does not deserve the attention he is seeking by blurting out. If this problem is a continuing issue, he should receive the appropriate consequence for failing to abide by class expectations, as it sounds like you have already addressed this matter with him previously.
One thing I've resorted to when nothing else works is the Lizard. I have a stuffed beanbag lizard I got from...somewhere. When I call on a student, I toss him or her the lizard. You're only allowed to talk (except me) if you have the lizard.
A fun class can play games with this--I've had students swipe the lizard to shut another student up, or because they want to add something. I've also had students 'call on' each other by tossing the lizard to another student.
Sure, it's silly, but it works.
I'll assume that you have spoken to him 1-2-1 about his need to not interrupt. I assume he is aware this is something you are hoping to change. So if I were you I would use Body Language.
When a student is speaking and you are focused on them, if Butt-in-boy pipes up, do not look in his direction. Without jerky movements or signs of feeling irritated or threatened, just calmy hold out your arm with your palm flatout and pointing at him (like a police officer indicates stop to a car but not so stiff or military). Continue focusing or the original speaker. This says "I'm a brickwall. There's no space in this conversation for you". This technique is also called, "Tell it to the hand, cos the face ain't listening!" :-) If necessary encourage the first student to continue or reply to them. Shut down his interruptions without them getting any reaction.
Or, an extreme tactic, you could issue him with 3 'speak out' tokens per lesson. Each time he speaks out it costs him a token which you take off him. If he has no tokens left, he can't speak out. It will help him focus on his blurting.
I used to constantly butt-in when I was in school, I was just showing off and liking the attention.
Most importantly though: don't let it show that it irritates you. Stay cool. Destress on Butt-in-boy and his interruptions.