I'm an elementary teacher, so some of the techniques that work for me like sitting kids out of recess, won't work for you.
Still - there are a few things you can do.
Have you considered calling the parents? What about changing a kid's seat assignment - moving the student away from the students that he or she was talking to? If the behavior is truly disruptive, have you sent kids to the office?
Your classroom rules should also be consistent.
If you keep telling Johnny to be quiet but don't impose a consequence, this tells the student that you're not being "serious."
Consequences should also be consistent. If Johnny continues talking and you tell him to see you during lunch while Mary who has also been talking gets moved to another seat - the underlying message the students get is that your consequences are capricious.
You may also want to check with your other 7th grade colleagues. Having rules that are consistent from class to class would probably be easier to implement since the students would (in theory) be familiar with 7th grade teacher expecations.