You can write a simple mail to them or else issue them a letter. A simple format is as follow:
Name of Employee
Sub: Late coming to office
This is to make you aware that we are receiving many complaints about your reporting time to office. Due to your misconduct of Rules & Regulations, you may be suspended from your respective job role from to .
Please consider this letter as a final warning.
Request you to report back to office on at am.
Name of Authorized Person
For Company Name
Thanks & Regards,
19th February 2011 From India, Pune