Hi Neha,
It is surprising that you have chosen to issue a Warning Letter to the Process Manager, whereas it is evident from your post that the actual offenders are the Team Members. Are the Team Members not your employees? Are they on the rolls of a Contractor or Third Party?
A Warning Letter issued to an individual is a serious matter and should not be dealt with casually unless there is conclusive and proven evidence of the employee having committed misconduct.
Warm regards.