Disciplinary action - Can the Principal employer issue any warning letter to the temporary employees for misconduct?

As he is an outsourced employee, inform your observation and recommendations to the contractor. Any warning has to be given by the contractor, not buy you as "Principal Employer". He is employed by the contractor, on behalf of the principal employer. You can warn verbally but not inwriting. That part has to be done by the contractor. you can let the employee know what you are going to do, if he was a good employee.

Thank you every one for giving support and awareness in my concern.

From your post it seems that you have hired a contract employee as a temporary employee.
In that case the Principal Employer is the person hired by the original company.
Your company is the hiring company from the Principal Employer. The Employee is hired for a specific period as per their Purchase Order. Normally the employee Collect the Purchase Order and handover to the Principal Employer.
As long as this principal method exists you can give a notice to the employee with a copy to the Principal Employer. A copy is handed over to the employee and get it acknowledged by him with date and time. This is sent to the Principal Employer immediately.
By this the Principal Employer knew the action from the Contractor who has a grievance and notice given to the employee for the misconduct or work ethics.
This will suitably come to action for monitory or verbal warning by the Principal Employer.
At times the employee is changed and replace with a new employee and a New Purchase Order in his name is requested.
In every case the Time Sheet is to be filled and mentioned there as the final day work..and aciton for the stoppage of this employee.
Because Purchase Order and subsequent Time sheet are relevant document for submission of Invoice by the Principal Employer to the Company for the services rendered.

who to prepare the explanation letter for usage of mobile in the office