What you said
As soon as you notice someone talking, warn them that you've noticed and tick their name on your list. They will protest that they were not the only person talking out of turn. Agree with them but point out that they were the one that you noticed first and you'd be happy for them to give you the names of other culprits. They won't, as "grassing" is a cardinal sin with pupils. Start ringing parents of pupils who have the most ticks over a week.
As well as inviting them to share names with you, tell them that if you are stopped by the police for speeding, you can't get out of it by telling them that the guy in front of you was speeding, too. It has worked for me in the past.
The expert view
Almost every teacher has gone through this, and will until the universe succumbs to entropy and chaos. You are new to them. They do not respect your authority.
But they will. You are doing the right thing: laying down the law, setting the boundaries, then enforcing them. But you must keep it up. It takes longer than a few weeks; it takes months for some classes, and a whole year for a few. But eventually they will see that you mean what you say. If you give up, they will renew their disobedience; if you persist, the clamour will dissipate and you will be gifted with another classroom entirely, one governed by law and structure.
It seems far away now, but keep setting detentions, make them severe, make them boring and full of hard work. Make them places no sane pupil would want to be, make the class learn that you will not be crossed, because you care about them and you know what the rules need to be in order for them to learn. You'll get there. Good luck.
Tom Bennett's latest book, Teacher, is out now, published by Continuum. Post your questions at www.tes.co.ukbehaviour.