I have a thread on
www.guild-hall.net forums in 'the outpost' that is stickied on the top just for this purpose. At the time of this posting guild-hall forums are down so Ill just give you a quick one here.
1) go to
http://support.guildwars.com/
2) create or login an account
3) click ask a question
4) under department do "violation report"
make sure you include the following in your post:
1) offending characters name
2) Time the problem took place (include your time zone, if you dont have an exact time provide a ballpark)
3) The character you were logged into at the time
4) a description of the problem
GW support team will usually respond within 24 hours, usually much sooner. During the day hours response time seems to be at less than an hour.