What do you think about splitting the scam rating, with a "warning" rating for scammed previously OR you strongly believe that they will scam in the future, and a "scammer" rating for scammed previously AND you strongly believe that they will scam in the future? And then if you only have warning ratings, the indication displayed next to posts will be softer.
I think this is nothing more different than just using a neutral rating for such cases, it is more softer rating like a "warning" rating as you described. It just needs to be use at the instances, but due to the lack of forum endorsed rules to use them, a neutral rating is often replaced by a direct red, even if sufficient evidence is not provided.
Now it is getting larger and in some ways unmanageable in the same ways, and frankly this obsession with not wanting any hard rules is hurting this community, not helping it at this stage. I don't think setting up a standard of evidence of theft, violation of contractual agreement, or violation of applicable laws for leaving a negative rating is at all excessive, and in fact is quite minimalist. Furthermore as I defined it would actually allow people to have a fair overview, not just a summary execution by suspicion and its over with.
I see this as a very valid statement as the forum has truly grown to a vast extent after some of the current rules were applied and it surely needs a new set of rules to manage the overgrowing community and at the same time scammers who try to attack in any way they can. Its not a bad idea to put more strict rules over a large community as it keep it bonded and productive.