I couldn't disagree more with folks who say it should be the responsibility of the PvE'r to avoid being unintentionally flagged for PvP on a PvE server. It is utterly counter-intuitive--it's a PvE server.
I recognize that there are players that want to do both, even on a PvE server--if a player wants open-world PvP, that's fine, they have the toggle.
Don't want unflagged players aiding flagged players? Don't allow targeting of, or aoe effects on, flagged players.
If there exists a possibility of griefing, it will be exploited; that's a sad reality of MMO's.