In my point of view, the only way to handle a bully is to give him a taste of his own medicine.
If I were you I would punish anyone who initiates an attack on any Newbie to temporarily lower his level to be 2+ level under the newbie (caped at level 1 of course). And to add salt to injury you should mark him so all Newbies would know that he is a Newbie killer. Let see if he likes being ganged up by Newbies
Unless you give them a harsh reason not to attack, nothing can stop them. They are not playing to follow the rules of real life. It’s a lawless life and they will do whatever they want. That’s the whole point of joining