can I make a doctor pay his accumulative bill if he did not sign anything?

I own a small denture lab business and began work for a network of doctors 4 months ago. Out of $18,000 worth of work, they have paid only $4,000. We finally wrote them an email explaining that we cannot recieve or do work for them until we are paid a decent amount so that we may cover our own expenses. The owner called me today and threatened me saying we can do this the easy way or the hard way. I do not see how he can attempt to make me feel guilty when he is obviously on the wrong. My issue is that my business has been running on a trust basis where my other six doctors have been paying for many years on time each month without signing anything except occationally the prescription that explains the details of the work we do. Can we make this particular network pay if they did not sign anything?