Thats why you shouldnt do any larger jobs over these sites. Do smaller scale projects over them to build your profile and reviews, and each larger project should go directly through wire or any other kind of payment processor, without freelance sites being middleman.

Just a tip, but maybe you should try to put public pressure on them. I had a case once when I got scammed for $1200 on Elance by a guy doing chargeback frauds, and Elance locked my account because I didnt had enough funds loaded to cover up the scam cost (they wanted to minus my balance by $1200 to cover chargeback, which I had no right to appeal to). I was locked out for a months, until I found a relatively popular blog post about Elance (over HN too, coincidentally), and an Elance representative who commented on it in comments. I replied to his comment and asked him why they did what they did to my account, and got account unlocked in less then a day. He probably figured out 1k$ bucks is less worth than negative publicity.