Tricky. I think a firm dissuasion email might make them budge. Something stating, for example:
- The facts
- The fact that your customers started receiving emails from his startup
- The fact that what he's done is dirty
- And that you're contemplating a couple of serious actions, while staying vague.
The idea is to make him react and open dialogue, because the thing is, I don't think you have the resources for an actual legal action, so an off the table deal might be your best shot.