As soon as domain is joined by the esxi then if in your active directory a group called "ESX Admins" is present then whatever the users are their in this group will automatically get the administrative rights on the esxi. So i am going to tell all of you how you can change this default behaviour that's given in this picture and You can even completely disable this functionality by setting the option Config.HostAgent.plugins.hostsvc.esxAdminsGroupAutoAddto false.
Removing the Administrator role from the group is initially successful, but rebooting the ESX/ESXi host grants the Administrative role again to the group if you will remove this group from esxi by directly connecting it. So the best practice is to change it to no access rather than removing it.
Hope this is Helpful for Everyone......Enjoy