There are times when you may have a user account that has an email address that you don’t want listed when people browse through their Outlook contacts in the global address list. There are many reasons to remove an email address from the global address list, but typically this is because you have created some sort of service account that needs to send mail through Exchange 2010, but not have the users see the email listed in Outlook.
To remove an email address from the global address list (GAL), start the Exchange 2010 Management Console, navigate to Recipient Configuration -> Mailbox and find the user who’s email account you want to remove from the global address list. Double click the user (or right click -> Properties) and at the bottom of the General tab, select Hide from Exchange address list. This will remove the users account from the GAL in all users Outlook client.