I'm not really sure we need to limit PvP here. Eventually, the concept becomes pretty self-policing. LotR, while an extraordinarily lethal game, was especially so when it came to matters of PvP. As a matter of fact, the climatic Battle of Antioch actually stands out in my mind because, if memory serves, every general was actually able to come out alive, and this was only because the dice were exceptionally kind.

If you get into PvP, avatars *will* die, that's a fact of life. I think that concept alone will prevent people from engaging in a war just because they feel like it and compel them to work in more subtle channels to get what they want.