In my forums I allow for the users to edit a post for 1 hour and do not allow them to delete their posts.
As far as deleting an account I made a group called suspended and all the permissions are set to 0 or -1. I then wipe the account information and change the email address to one of my crap email accounts, turn off all notifications, and remove their avatar and just leave their name associated with their posts. I only do this if the user request their account deleted.
For permentaly banned users I pretty much add them to the banned list, put them in the suspended group, and turn off all notifications to their account. I only ever had to ban less than 12 people since 1998.
