[Lady-Loki]

I doubt it's technically against the rules because if you level while you are in there you don't get automatically warped out. So, if he leveled and never left, then I guess he is technically legal although theoretically wrong. It would be easy enough to do as long as you have access to a second PC and a separate account so you have someone to come in and get your drops to NPC (because, why else would anyone want to stay beyond the level since they probably could get better exp someplace else?).

Try getting some screenies and send a PM to a GM and ask if there is anything that can be done and maybe they will warp him to jail to explain that it's just not right to occupy an abyss designed for lower levels. Or, if you get him killing lower levels thay may get him for harassment and interfering with game-play.