From the Model Mayhem FAQ:

What can Model Mayhem do to help me resolve a dispute with another member?
You are welcome to contact us to report a dispute you have had with another member, from contract to payment to image to personal.

That being said, however, please keep in mind that Model Mayhem does not and cannot arbitrate these kinds of situations due to the often subjective nature of disputes. We will only be able to make a note of your report.