Ticketed a player, redirected to the forums, which told me to make a ticket...
I opened a ticket after someone ninja looted items the other day, within the ticket I included screenshots which I believe supported what I said, and showed that someone ninja looted items, I then got a reply in game from a GM telling me to look at how to report ninja's on the forums, which clearly tells me to create an in-game ticket with screenshots, like I did.
You can do the report either in-game or on the forum (via PM to a GameMaster), but make sure to stick to this format. Needless to say, the image or video proofs have to be clear.