My suggestion is to have a standard of evidence of theft, violation of contractual agreement, or violation of applicable laws before leaving a negative rating. This solves the primary issue of there being no limiting factor to false or frivolous negative ratings with minimal effort, creates a simpler system, and less counter productive side effects.
Forgive me if you've already answered this, but should this be implemented what would make people follow these rules? Would someone have to sift through every negative rating left and verify that each one is valid according to them?