Something which has been in discussion from time to time but which lacked a developer with time to implement it, is avaible via Credence (which sadly was a university project and died when the students thesis was done):
Credence: Thwarting P2P Pollution
Credence does decentral spam marking and detection, and if it would be implemented in Phex, it could give Phex a real boost.
But I assume that it will be quite much work...