Telling somebody "you've been banned" will make them upset. Some people get severely offended and they want to fight. So they go and create new accounts and start over, or they seek revenge in other ways. It can be very time consuming to deal with. Granted, this will still happen because some people will realize that they've been hellbanned, but the ones that simply get the impression that people aren't responding to their trolling will get bored with it and go away. Which is a win.