Ask the people at your consulting company to deal with it. Generally, consulting companies have a "no free work" policy that you and their clients must abide by. When you have a company that wants to treat you like a salaried employee by working you overtime for free, tell them they need to take that up with the consulting company because you can actually get fired for working for free. Sounds odd but, as you know, consulting is a whole 'nuther world where no one get's anything for free. I believe you'll find the "no unpaid overtime" time clause in the contract with the Client, as well.
--Jeff Moden
Change is inevitable... Change for the better is not.