If an account has administrative access to a particular computer, you can only make it difficult for him to access certain portions of the file system, you cannot however prevent him from accessing anything. This is because as an administrator of a box, they can always take ownership of a folder and then alter whatever security permissions you may have set to prevent access.
To clarify what I believe GENEnG is trying to say, don't make the remote admin an administrator, instead provide him only the permissions he requires to perform his work. I recommend doing this with a security group that contains his administrative account so that you have more flexibility in the future. It's very possible that what he needs to do will require administrative access, in that case there are some steps you can take through group policy to limit his ability to meddle with stuff, but at the file system level, he will always have the ability to access any portion of it because of the "take ownership" back door.
Again, I'm not saying you can't make it difficult for him, but if you give a technically sound support professional administrative access to your box, they will ultimately be able to do whatever they want regardless of NTFS permissions, Group policy etc.