Information
Enabling lockdown mode disables direct access to an ESXi host requiring the host be managed remotely from vCenter Server. This is done to ensure the roles and access controls implemented in vCenter are always enforced and users cannot bypass them by logging into a host directly. By forcing all interaction to occur through vCenter Server, the risk of someone inadvertently attaining elevated privileges or performing tasks that are not properly audited is greatly reduced. Note: Lockdown mode does not apply to users who log in using authorized keys. When you use an authorized key file for root user authentication, root users are not prevented from accessing a host with SSH even when the host is in lockdown mode.
Note that users listed in the DCUI.Access list for each host are allowed to override lockdown mode and login to the DCUI.
By default the "root" user is the only user listed in the DCUI.Access list.
http://pubs.vmware.com/vsphere-65/topic/com.vmware.vsphere.security.doc/GUID-88B24613-E8F9-40D2-B838-225F5FF480FF.html
http://kb.vmware.com/kb/1008077
Solution
# Enable lockdown mode for each host -
Get-VMHost | Foreach { $_.EnterLockdownMode() }