With all the above stated, a consumer lawyer would likely be interested in talking with you if you have evidence you DVed the CA, but the CA persists in their collection efforts.
Here are two sources for consumer lawyers.
If you're still getting dunning letters from a CA, dated after the date shown on your RRR green card, that's pretty compelling evidence the CA is willfully violating the FDCPA. Letters are pretty easy to present into evidence, and they are quite damning.
If you're still getting phone calls from a CA after they received your DV letter, those are a violation. However, you're going to have to try and prove it. Caller ID is your friend, and a photography of your Caller ID unit showing the incoming caller's phone number is quite compelling. But some CAs spoof Caller ID, which isn't illegal today under federal law. You might have to consider getting a telephone recorder and taping your calls.
Your best bet if the CA calls, and you recognize their number on your Caller ID, I'd just let the answering machine pick up. You don't have to give any sort of notification or obtain consent, and if they are calling and leaving messages that's compelling evidence they are still trying to collect.
You can sue the CA and collect $1,000 plus attorney fees. But the real payoff is if your lawyer can show willful noncompliance, then punitive damages kick in.