Well you need a screenshot of said scam, then go to
www.guildwars.com/support and follow the relevant link there. Once you get to PlayNC support, ask a question, giving the exact date, time and location of the scam. DO NOT, I repeat, DO NOT ask for lost goods back. PlayNC is incapable of restoring stolen/lost items or gold, and asking may result in you question being sent to the Recycle Bin.