what if we register an account at the bitcointalk forum and are banned because a violation has been detected.
Accounts never got banned after register. You are incorrect.
If one account created with an evil IP address, it will be restricted (not banned) in only-read mode. You can read but can not make posts.
The message is:
You are unable to post. How to enable posting.
Not only your real IP address but also address you have by using VPN, Tor can be evil ones too. So if you are serious to use your account (with your favorite username), the best way is to pay a small fee for evil IPs. Since years ago, users were not allowed to change username, despite of VIP members and very special cases.
The fee will be different and depends on the level of evil that IP has. Generally, it is not a too high fees AFAIK.