Let me explain why I want to do this. If a user is banned by the conventional means, they lose access to utterly everything, and their profile becomes inaccessible by anyone except those with "bypass user privacy" access. This wasn't good enough for us in most cases, because we have a ban appeal forum to enable banned users to communicate with the forum staff - users get moved to a "Banned Users" group instead. However, this means that the banned user's profile remains open to access and permits users to continue posting profile messages on that user's profile. I haven't found a way to change this functionality off the top, so I presume I would need to write a plugin to do this for me?