A letter from an attorney threating a counter suit for copyright violation and explaining the penalties would probably get someone to back off.

Only once have I sued for non-payment. I won in small claims court and put a lien on the guy's house. I did eventually get paid.

>I had one guy telling me all the ways he was going to sue me for not giving him (not fully paid) source code.
>"Uh-huh"
>"If you have the money for lawyers, you have the money to pay me."
>He came back over a year later with money, and all was good.