All I am trying to do is reject auth requests from a specific realm.

http://wiki.freeradius.org/Operators

I am not sure what are you trying to do. If you are trying to match regex
then use a proper operator. And if you are trying to do an exact match
than right side should be a string not regex. And use just "Realm" on the
left.

So something like? This gives the same error.

if (Realm == your.realm)  {
    update control {
        Auth-Type = Reject
    }
} 

Where is the syntax for flow control like 'if' documented?