Ideally you need them to come up with a list of class rules and consequences and post this on a big piece of cardboard in your class. If the bad behaviour has started recently and you don't have any classrules, you can say 'I think that this class can work much better. In groups of 4, I want you to come up with 5 rules of acceptable behaviour'. Give them 1o minutes. Then get a student from each group to write up their suggestions on the board. Ask the students to vote for each rule by raising their hands. Get the student with the best handwriting to write your class room rules on a big sheet of paper and display it in your classroom. Your school should also have some classroom behaviour rules. If you involve the students, then you can remind them of 'their rules' that they agreed on . Ask them to copy down the class rules in the front of their books. Don't allow any mobile phones or ipods.
If a student's misbehaviour is constant, I suggest that ask another teacher for advice who has taught that student before or that you ring their parents.