I completely agree with you about this, part of the reason I buy my premium membership is because I want to sit with other Dees fans. The St Kilda game was the one that really got to me, with a bunch of loud and obnoxious Saints fans behind us. They started goading from the first bounce but it took until the last quarter for them to be thrown out, turns out that they were actually there on sponsors tickets. I don't want total segregation, but people need to be mindful when they sit in that area that they are sitting in an opposition supporters area, if they want to carry on they can go and sit just about anywhere else.
According to the guy who runs the Redlegs (terribly sorry can't remember his name) there are no rules against bringing in opposition supporters, but there are designated bays where you are supposed to sit with your guests, I think this is the last two bays on level 2 where the Southern Stand meets the Olympic. The other thing to do is email the club and complain, if we don't let them know that we're unhappy then nothing will change.