...\GFI\MailEssentials\Attendant\data\usermanagement.xml (MailEssentials 2014 R2 and later)
Backup the xml file before doing any editing. Locate the following tag in the file:
<User>
Remove the User group entry of any account you do not want to have access. The SID listed is the Windows Security Identifier. If you are not familiar of which accounts these SID's pertain to, you can visit this link for more general information: http://support.microsoft.com/kb/243330
Re-start the GFI MailEssentials Services and check the Access Control List
