My next-to-last-resort option - which has worked every single time - is to spell out the scenario, very professionally, in an email and CC the offender's immediate superior, even if that superior was not part of the negotiations. I've even CC-ed VPs and CEOs depending on my level of frustration. It's amazing how fast things get fixed when you expose such behavior to their bosses. In my experience, most higher-ups have a vested interest in the professionalism of their company. You may do any more work for that particular person, but you'll probably get paid. However, I've had projects where the CD got pulled from the project over his exposed behavior and I still got more work down the road.

I once had a client decide to change creative direction and hire another company half-way through a project I was doing for them, but the CD didn't have the courage to tell me, so I kept working. It was an hourly job so the billing was piling up. When I delivered the final, they informed me that I had not been needed for some time and didn't want to pay. I got the CEO involved (through my CC-ed email response) and he started communicating with me directly. He wanted me to take 50% payment for what I did since they wouldn't use any of it. I was professional, however, and pointed out the obvious that they should have informed me right away of any change in direction - I would have stopped work immediately and bill them for work to date. I told him I still require full payment, but that I would credit them that 50% amount on the NEXT project they do with me. He thought that was a very professional way of handling it because they still would get that value of work. FWIW, I have yet to do another project with them but I did get full payment quickly. 
Professionalism is the key. 

Andrew Johnson

iMed Design, Inc.