Recently I had the following idea while I was banning a user (potential bot): I was thinking about 2 more usergroup permissions:
user is hidden: this will hide the user from all where users are displayed. E.g. newest member, memberlist, currently active users etc. It should hide posts of this user too, like "in conventry" (or make this as an extra option). Such feature would be very helpful for banned users or a bad-bot-usergroup.
user is soft deleted: it is nearly the same but a little bit different for it has the same effects like on a hard deleted user but the record of this user keeps in the user-table. Users will be displayed as a guest in threads but their posts are still shown (or not, perhaps make another option to different this ).