Is spawn camping a player that has Warmode toggled on, a challenge that invites PVP, a bannable offense? I was looking over the Griefing guidelines and I don't see a rule that is against this practice.
Griefing refers to the act of intentionally ruining the experience of another player beyond the normal gameplay loop. To ensure the game remains enjoyable for players of all different levels and types, we have forbidden a handful of behaviors:
Farming quest givers, flight masters, important quest objectives, or otherwise important NPC's for long periods of time and blocking other players from accepting or turning in quests.
Impeding a player by following them around and tagging mobs they need to kill for a quest.
Kiting a group of mobs into another player's (AoE) spells in order to kill them.
Interfering with any player trying to complete The Balance of Light and Dark or the Stave of the Ancients quest chain.
Dropping infernals and doomguards on a group of players in low level zones.
Killing a mob that a hunter is actively in the process of taming.