They ban and they come back. It's a never ending war on MMO's with these type of things.
Only solution I can think of is don't allow chat/trading until level 20. Would die down quite a bit.
@GamerKillance No, not really, just doing the beginning tutorials bumps your character up to lvl 21.
This. If you restrict the ability for bots to do <thing> until Level X, they will simply level all of their bots to X so they can do their <thing>.
Honestly, it becomes more effective (and more impactful, though not as noticeable) if they ban the BUYERS instead of the sellers.