This might be too much custom content, but if Warnane would consider it, I concur. It would be interesting to introduce an initially small penalty that grows each time you kill a player that does not yield honor.
I cannot, however, imagine what that penalty would be. Losing gold seems mundane. Hmmm... Perhaps the ganker becomes debuffed with an increasingly long ressurection sickness! 1st time; 10 minutes, 2nd time, 20 minutes, 3rd time: 40 minutes, etc...
Ooh, that's good...
But it might be too custom.