So obvious in hindsight, but it had me fooled. Found this on another forum:
The spambots are gaining intelligence. In Auridon we encountered several who would create enough space in their message that their own name was invisible, without enlarging the chat window; later, a second bot utilized a clever method to create a false-name in the chat window.
If you can't right-click a name to ignore, scroll up a bit higher. The bot is generating space, and then typing in a decoy name -- it's just regular text, but it looks like a second user. In this instance, the user was something along the lines of ATMfourb or some such nonsense, but it threw out [superman] as a mask. Users were trying to ignore [superman], only to discover that right-clicking wouldn't work.
If you encounter this, scroll up a bit, and look for the previous poster (likely using a canned message -- in this case, '***?'). Ignore and report THAT user, and we all win.