Mine is still in the newer stages so only ones I've banned were spammers that were all over Proboards. I haven't had to put someone on 'probation'. But I would give them the benefit of the doubt.
Yes, it's a must-have to have a banned group rather then remove or delete their accounts. If you deleted them then all that information be lost and they can join again and start the fuss over again. When you ban them it remembers and also with IPBs they can be added to the bad blacklist and so other IPS sites can look out for them.
Most of the ban members on my end are just fake or bots, I only had to ban 3 real people and they haven't been lifted since then. End of the day I have a system where if they break so many points they are OUT. after 8 points they are ban. Meaning to get all 8 points they had to be getting warning points, 24 hours ban, accounts restricted and you name it.
Yes I currently have a banned group so that way I can keep track of who was suspended, for what reason, while also keeping their content online and for them to be able to see why they were banned.
That is a good idea regarding having a section where only they can see and post into..maybe add that, or possibly link to the internal contact us page to send an email to my support email address.
In the past I've normally just banned people and left their accounts in that state. Preserves their content and keeps them on the system so I know what they've done