I understand ur problem. A letter will not work in this regard.
Do one onething mark a mail to all who invovled in it & CC to UR Deppt. Head.
Discuss the issue. Ideally ur Department head should mark the mail.
Next best alternative is , immediately arrange a small training session pertaining to HR policies.
Make sure if same cannot be repeated again.
M with u. Best of Luck!!:icon1:
9th March 2009 From India, New Delhi