There are times when negative feedback before a scam is invaluable. Recently, a trusted member tried to get a lot of coin based on collateral that she had private funds coming in this spring. She may have succeeded if I had not warned people, and I can always remove my feedback and make arrears should she actually show proof of funds. But there were so many contradictions in her story she has lied at least once already.
This is the most important part of "preventive" feedback. The user would have to be neutral enough to withdraw negative feedback if it turns out that it was left excessively. I saw earlier that not everyone was ready to admit a mistake and withdraw negative feedback.
It is often very difficult to prove a scam beforehand and I think that we shouldn't be easy on the trigger. Getting out of a negative rating can be a very tiring and long process, certainly demotivating for services to stay here.