Thanks for this information. The Paket Filter ist able to drop communication between the members of a group but it does not prevent them from having each other on their rosters.
I'm well aware, that Openfire depens on the spare-time efforts of volunteers and I'm grateful for their work. But as I have not the necessary time and knowlege to patch the source myself I can only point people on what functions might be a nice addition in the future. And don't worry, I don't expect immediate implementation of this feature. 
Jörn