He can go public and warn folks if after three written interactions they acknowledge that he delivered the work but refuse to pay or give a timeline for payment. Has the same effect as spending a lot more time on a client that will never be a reference.<p>Also, read the book, there are a number of things you can do to manage your "credit policy." Essentially you are loaning the customer N hours of work: if you look at it that way you may engage differently.<p>Finally, life is short and getting screwed out of 100 billable hours is not the worst thing that will happen to you even in a two or three year period.