I've been looking out to do the exact same thing you're looking for.
Unfortunately this is by design as of the existing version. The closest I've
found whas at this URL:
http://www.anetforums.com/posts.aspx?ThreadIndex=3115
I think that Microsoft should change Remote Assistance to be able to be
configured by a GPO when it comes to if users needs to accept a connection or
not. Why?
Well, then you could, as being the administrator, use Remote Assistance
conveniently for two main reasons:
1. Quick and simple connection to the end users desktop to support and
maintain users software
2. Monitor user activity when the user is suspected to be involved in
illegal activities on his employers computer.