Rejecting auth from a specific realm

I am using freeradius to proxy eduroam requests. These could be for any number of different realms so I only have a DEFAULT realm configured. I now want to reject authentication to one specific realm (my own) but pass all others. The proxy server can't do this for me so I need to do it before proxying. I have been reading all the man pages but can't figure this out..how where is this done? B