You can't just ban people though. Some scam reports are just a misunderstanding while others are complete bs. I do understand the concern but just banning anyone who has a scam report is a bit much.
I've seen one nother site where they put limitations on a rank (like TWC) where you can't access the marketplace or send/receive PM's. like you said, this would be a good start though they can post elsewhere. I'd just make it so that they can only post in areas like the scam report or spam section - only if it's a valid scam report.
Lastly, you can't protect stupid. There are just stupid people wherever. They do what they want because they're stupid and it's like a disease. People can have 50 negative feedback, a TWC PiP and be registered today and will still scam just because there are stupid people. I can't blame the ones who fall to big names because there's almost nothing you can do to prevent a scam quit, but there are obvious signs for imposters and random people who are trying to scam. You just can't help these sorts of people and you're going to go nowhere trying to. I know it's unfortunate but you can't help people who don't care.