There are worse things than being banned, for example, if the community came together we could put together a police force that responds to high level Gankers killing Lowbies, show up and take out the ganker until he gives up. If the community banded together we could blacklist these players from being in guilds.
A collective shun of a ganker would be far more effective than a Ban, Bans they will just change IP and create a new account. big deal, Actually being known for having a bad reputation would be far more effective, and this is something that the players can do.
Heck we could even right a Add-on that tracks and displays a persons reputation in pvp and pve, and have it do minuses to the rep if it sees a pk with no HK, enforcement isn't the problem, its if the community wants to enforce it or not, that is the problem.