I don't know if you're unaware, but we currently have this exact system. If you wish to investigate a user you can search up their name in
Report a Scammer Archive and
Dispute Forum Archive, you will find any previous offences. This is specifically why we do not delete or remove unique identifying information from threads, they can be used to assess someone in the future.
As for your suggestion, the pardon system is a clean slate. We want to promote users to amend their past mistakes so that the victims have a chance of getting their money back. Having a mark on their profile will discourage users from making things right and will encourage continuous ban evasion.