Dino, I hate to tell you this, but there are different kinds of poachers, and this guys seems to be the worse kind. His motivation is money. He has no concern for the plants at all, he won't be happy until he has made every dime he can from these plants. There is nothing that you can do that'll change his mind. So all your letters will do nothing but make him laugh. The only way to stop him is to somehow outsmart him.
Remember this guy is not convicted of any crime nor is it known for sure if he has broken any laws.