I've seen it happen before where guests were able to type in chat (not sure how), but they're suppose to only be capable of viewing the site, not participating. If the person had a username then they must have been a registered member─though it sounds like they should be registered as something else.
But yeah...that's weird.
If you remember the name, you can still report it. It's better to report the person while the incident is fresh. Hopefully you don't have to go through that again.