How to restrict specific developers to checkin files in main trunk?

[Kudiyarasan]

Hi ,
    Now my  position is  --All developers can check in files in main
trunk except few . And  that few developers can only check in files in
branch trunk , they are not allowed to check in files in main trunk .
i.e how can I restrict main trunk but branch trunk  ? . How can it be
done ? .

 Thanks & Regards ,
 Kudi