“Griefing” in and of itself is a little complicated, Christmasday, and generally means different things to different people. I’ll focus on the act.
You may, through PvP, kill the same character repeatedly. As long as they are flagged/in Warmode, and able to be attacked, it is usually not considered a violation to do so.
If there is some kind of bug or exploit that allows you to abuse a system, that might be something else entirely. I’d limit how often you “test” a possible bug so it does not look like you are trying to take advantage of it.
If you are colluding with the enemy faction that can fall into the area of win trading and would usually be considered a violation of our policies.
I hope that answers your questions. 