Activity log checks PVP
I think Haypi should review the activity logs of let's say the top 20 players at points throughout the PVP competition. Its pretty obvious there's a few players who feed like crazy.
Even deterring this kind of activity is better then nothing. PVP is a great concept but the feeding is everywhere. It's easy to spot all the feeding cities with no stats and always fluctuating in prestige back and forth.