I'm quite tired of seeing posts on the feedback section about players who have been betrayed or lost loot to PvPers asking for a way to counteract this or a way to report these players.
Pirating is not against the rules and the alliance system was designed to work this way in which betrayals are always an option.
Unfortunately these posts gain quite a lot of traction from other people who don't fully understand this and people informing OP that this is part of the game even if 20 other people have already said so.
A sticky post at the top informing players that this is not the place to complain about loosing loot would deter this a lot and inform players that the alliance system works as intended