How to prevent users changing details in Contacts List

[Satish Boddapati]

Hi All,
After entering the contact details in the contact form, How to
prevent users from changing the details through the Conacts List ??
We have customised the contact form for not entering the
non-codepage 1252 characters into the form. But, there is a possibility
that user may do the trick by entering the non-codepage 1252 characters
into the contact details through contacts list by clicking on a
particular record with the help of a mouse and directly editing there.
As per our requirement, the contact details shall be synchronised
to Siebel through Exchange Server.

Please suggest me how to avoid users editing the contact records
through Contact List.

Thanks & Regards
Satish Boddapati