"We simply want to let our members know about this theif and try to at least help ban the theif or just change the theif's point of view in stealing. "

With that, you have defined the purpose of "anti-art-thieves" and re-iterated my whole problem with this community.

Why have them banned? That doesn't solve the problem, at best it sends a message to them that having the simple desire to be artistic is wanting to be a part of a very exclusive club with strict rules and a hierarchy. That's not the world of art, by a longshot. Art does not have rules, and it does not have a hierarchy. Art is EXPRESSION, and nothing more.

If someone sees your artwork, and likes it enough to think that they could visualize it better (or to suit them), that's not theft, by any means! The result is you've enticed them explore their creativity and this is their first step. They're basically using your example to explore creativity.

If you assault them with accusations and threats and a ban, that just sends a very negative message to the supposed "art theif", and most likely they'll walk away from this experience as either someone who has become apathetic or your enemy.

Suppose you approached these individuals in a different manner. Tell them that you noticed that they copied your artwork, and then encourage them to take the next bold step by educating themselves by practicing, learning, and drawing for themselves. Take the higher ground and be a positive influence... and that's all I have to say about that.