When trying to remove the check mark next to "Enabled for Lync Server" under "Lync Server User Properties" menu the following error is displayed in the web console:
Invalid data entered on this page.
View error messages under input fields, and correct your input.
SIP user name must be specified.
To disable a user for Lync Server, please ensure to have the "SIP user name" field populated with an applicable user name. Once populated proceed to remove the check mark from the "Enabled for Lync Server" check box.
If removing a user from Lync Server plesae use the command "Remove From Lync Server", this will completely remove the user and its Lync Server attributes from Lync and Active Roles.