Here you can not delete users but you can remove permission (because these permissions are defined in Global Permission cannot be removed from here so that's why remove button is grayed out)
In order to delete users/groups:
Click Home, and navigate to Administration > Single Sign-On > Users and Groups. Select User tab and domain.
In the list of users, select the user that you want to delete and click the Delete icon.
You can delete users that are in the vsphere.local domain from a vCenter Single Sign-On management interface. You cannot delete local operating system users or users in another domain from a vCenter Single Sign-On management interface.
If you wish to delete AD users you need to do delete them from your domain controllers. but if you want to revoke their access to the vCenter Server you need to remove their permission.
Since you havent CREATE the AD users on vCenter so there is no reason to DELETE AD users on vCenter. You just have to remove the permissons.
They delete function will be work for users you have create on vCenter within the SSO domains like vpshere.local for example.
Regards,
Joerg