I’m not sure what the laws are in your state, but it sounds to me as if they have no legal authority over you. The local real estate board is there to regulate real estate agents. You’re not a real estate agent so you don’t fall under their jurisdiction. After all, one of their powers is to take away your real estate license which you don’t have.
Realtors are supposed to follow a code where they’re not supposed to steal customers from each other. If you really want results, I would suggest giving up on sending the letters and try to go to each house individually to meet with the seller.
In Texas, you could file a complaint to TREC and the agent could get fined or suspended for making unfounded legal threats.
If the property is not under listing contract, then the broker or his agents have no legal rights. Having that said however, make sure you are marketing to expireds not Cancelled listings since cancelled listings may still bound by a contract.