I have now sent another letter to this place asking them not to call again, and mentioned that I've already sent them one letter, and that by them continuing to call me repeated times a day, they are violating the FDCPA.
This time, I overnighted it, instead of certified mail...b/c they still have to sign for it.
They called me again this morning, so that makes at least 3 calls since receiving my initial letter...