Well, the way I solved a similar problem with a sales person was to send him a mail in which I made clear that I was about to contact his ex-employers to invest

Well, the way I solved a similar problem with a sales person was to send him a mail in which I made clear that I was about to contact his ex-employers to investigate, and ask questions like "I intend to follow legal channels and I was wondering if you had similar experience working with X". After all, paying for unfinished job is at least reason for a trial, and good pre-trial strategy is to look for a pattern. Of course, you don't have to actually do this (though I was prepared to at the time), but hopefully you won't need to.