So, does anyone know how to disable access to Server Manager,
Remove the "FILERDOMAIN\Domain Admins" global group from the filer's "BUILTIN\Administrators" local group using User Manager for Domains. This will prevent your NT adminstrators from messing with filer shares.
I am almost, but not quite 100% certain that administrative rights to the file system will be unaffected by this (the "Administrators" local group SID is a domain-wide constant if memory serves, so it should still find its way into your NT admin's security tokens via other means), but give it a try.
Keith