WARNING: If someone report you ,you could get problem because blizzard told us that you can get a temp bann for killing/attacking quest Npcs.
anyways here it is:
1. Step onto the edge at the mailbox
2. Check "at war" with Argent Dawn in your reputation panel.
3. Start attacking them, they will go bugged and evade for a short time, and people wont be able to interact with them, and get marks/info about scourge.