Continued calls after C&D

I don't have an account number for the agency that keeps calling me. I sent a very long letter asking for validation, then asked them to stop calling. I did NOT sign the letter, just typed my name. (I don't want them forging my signature, which they've apparently been accused of doing on multiple accounts!) I don't have an account number, so I just left it with my name and address. They have been calling even more. I'm afraid that they'll say that this was not a legitimate letter, being unsigned and with no account info. What can I do? I DO NOT want to call them to verify anything to get an account number! I know that this is a downhill slide into even more harassment with this company. Any suggestions? Was what I provided enough? THank you!