Provide a new setting to forbid end user sending mail from his mail database

The customer wants to forbid some user to continue using mail function ( both send and receive new mail) but can keep on refer existing mails in his mail database.

Do the following can stop receiving new mail:

1) Delete [mail server], [mail file], [tranfer mail address], [internet mail address] in user document

2) set [mail system] to Nothing

But the user can still send mail to other user from his mail database.

I asked the customer to give only Readers right in ACL of the mail database, but the customer said that end user still wants to have Write right for mail archiving.

The only way to stop send mail is to use server side mail rule to stop pressing mail sent from the user, but for each user one mail rule should be created.

The customer wish HCL to provide a new function (new setting in User document or some policy) to forbid mail sending from his mail database.