I'm in an unfortunate situation where a client I'm currently working for is threatening to provide bad rating and feedback to my profile on a popular freelance outsourcing site (not sure if I can mention sites or not here, so hence the vagueness) if I don't help him with some other work that are not part of the initial project agreement and it's monetary value.
In simple terms, he want me to work more than what we agreed to FOR FREE to which if I don't agree, he will destroy my online freelance reputation that I've build for quite a long time with false allegations of incompetent work and bad quality rating and feedback.
Since most of the discussions I had with him is mainly via Skype calls, I can't provide any solid evidence of contract-breach to the freelance site support either so I'm kind of in a fix as I really value the rating percentile I have managed to maintain on the site.
What would be the best approach to get out of this predicament as reasoning with him doesn't seem to be working at all.