Do you think that a ban will solve the situation with these accounts? I think it is not difficult to guess that if someone is so stubborn, they will find a way to return to the forum with a new name but with old habits. Therefore, in this case, and all similar situations, there is no point in a ban. All accounts have negative tags, which are great at scaring everyone away from having any connection with them. A new account bought or created, but in the same hands, is much more dangerous than an old and well-known scammer.
I partially disagree with you. I've seen scammer (who have high rank and have some negative tags) add scam website on his signature and write non-spam post on board that doesn't show trust number. If those scammer are banned and forced to make new account, at least they can't use big-sized signature or attach image without additional effort (such as obtaining merit or spending money on copper member).