If it wasn’t mentioned up front, I think it’s poor form to try to retroactively apply it. I say you put him on the “not so good” list, and he’s on a cash prepaid in full basis for any and all work to follow. Meanwhile, keep sending registered letters he has to sign for, to create a trail in case you have to go to court later. At 120 days, turn it over to a collection agency or write it off for tax purposes.
If the guy is just a client, you have little leverage, he’s not planning on coming back, but if he’s a local producer that works with the same small group of vendors and you, you can sometimes use the leverage of reputation. Once everybody knows he’s a bum, he loses their cooperation: they may still take his business, but it will always be on unfavorable terms, cash up front, no credit, no releasing tapes without payment in full, no special discounts or favors, that sort of thing… and he will be miserable.
Understand, I wouldn’t go threatening to defame him in the production community, or public forums, particularly if he’s the litigious type. But there are ways people find out about who is a stand-up guy and who is not.
Something Leo T. once said bears repeating too (paraphrasing): everybody gets into some bad luck from time to time, and if you’re not 100 percent sure the guy is a crook, your better nature should be to find out if he’s just got a temporary cash flow problem, and maybe you can help him thru it if he would just send you a token percentage for now – until he gets on his feet… say, 20 bucks a month or a week, then the balance when he can manage it. When he’s back on top, he will surely remember you as a special friend, and perhaps more work will come of it. It’s up to you and your budget just how generous you can afford to be in such a case, and nobody can tell you exactly whre to draw such a line. Myself, I tend to be on the cynical side, having been burned often enough already. I give everyone just one free shot to try to rip me off, then it’s war. The minimal payments also help your legal case, should you need to sue later anyhow. The are evidence of a contract demonstrating consideration, is the legal term, though I’m not a lawyer.