You need to get his address and have a lawyer send a very sharply worded letter that lets him know that he's in your gunsights. Don't write this letter yourself

You need to get his address and have a lawyer send a very sharply worded letter that lets him know that he's in your gunsights. Don't write this letter yourself, or if you do, have a lawyer check it over before you send it. There are laws against making threats (even against someone who might be breaking the law).