If Lumi wasn't able to ban people outside of her chat, it would be easy for people that aren't fans of Lumi to troll her with Lumi-nugget artwork.
Right now though, this is the first time I've ever seen Lumi issue anything like this sort of ultimatum, and it's over one very specific request for artwork she doesn't want to see or want associated with her. If it was a more general request, or Lumi did this sort of thing more often I'd understand the complaint but right now I think I'll side with Lumi on this one.